Creating the "Perfect" Mass Effect Save
The idea of "creating your own story" was one of the building blocks on which BioWare attempted to establish its Mass Effect trilogy. The notion of events in previous iterations affecting later ones is certainly nothing new, but these events being entirely up to the player adds a lot of customization. So this begs the question, with no real "canon" to speak of, what is the perfect Mass Effect story? Well obviously no such thing really exists as the world your Shepard shapes should be just that; yours. However, there are plenty of things you can miss if you blindly rush through the first game, so this will be an attempt to list the changes ranging from the slightest dialogue nods to actual universe changers.
WARNING: This is meant to be more of a checklist than a first-timers guide. It focuses solely on the missions that directly factor into Mass Effect 2; this is NOT a "100 Percent Guide". There are still TONS of things to do in this game; they just don't have a direct impact on the sequel. Spoilers will be present throughout as this will be written under the assumption that you've already completed Mass Effect 1 and are merely attempting to create the ideal save for ME2 and ME3.
First off is your character creation. You'll be able to choose a gender, origin story, and a "psych profile" for your Commander Shepard. They all have a small quest or acknowledgement tied to them in ME1, but don't expect anything more than an e-mail to your private terminal to reflect this decision in ME2.
As Mass Effect 1 begins, you're more or less on rails until you arrive at the Citadel and start to assemble your companions. Side quests aside, the first thing you'll be forced to do is recruit Garrus Vakarian. After Captain Anderson informs you that a man named Harkin knows where you can find him, you can simply opt to head straight for the Medical Office and not speak to Harkin at all, leading to Shepard not knowing exactly who Harkin is in ME2. Next, although I'd highly recommend against it, it's entirely possible to meet Garrus and reject his offer of assistance and not take him on as a squad mate for the entirety of the first game. This also leads to a slight change of dialogue once you meet up with him on Omega in Mass Effect 2.
After recruiting (or not recruiting) Garrus, you are off to find a Quarian who has been in contact with an agent of the Shadow Broker named Fist. When you fight your way through Chora's Den and finally meet Fist, you're given an opportunity to either spare him or kill him. While killing him has no repercussions in Mass Effect 1, if you spare his life you'll find him in the lower levels of Afterlife in Mass Effect 2 where he'll greet you with a rather lukewarm reception.
After this, you're basically free to roam about the galaxy in whatever manner you see fit. However, there are plenty of sidequests to be done on the Citadel and throughout the galaxy. From here on out, I'll try to organize this into locations rather than chronologically. I'll start off with side missions and then mention the main quest.
One of the first side missions you can encounter is immediately after your Spectre induction. Upon descending the stairs you'll meet a Salarian named Chorban. After a dialogue, you can offer your assistance in scanning the Keepers spread throughout the Citadel. If you complete this quest, Chorban will send you an e-mail in Mass Effect 2 that concerns the rather grim revelations he uncovered with the data you obtained.
At the top of the Citadel Tower, right near the Council Chambers, there is an Alliance official named Admiral Kahoku. You can eventually learn that he is trying to locate a squad of his that has gone missing and offer to aid in the search. Completion of this will lead to a chain of side quests that involve Cerberus, the human-centric organization that has a prominent role in ME2. Completing this alters a good bit of dialogue amongst Shepard and his/her crew in ME2 when talking about Cerberus' checkered past.
On the Presidium of the Citadel, in between the Consort's chambers and the Emporium, a human female named Helena Blake will flag you down asking for your assistance in a pressing matter. Should you accept her quest, it will eventually lead you to a confrontation with her at a set location. Upon this, you'll learn the true nature of her prior proposition and have a choice to either convince her to change her ways or kill her. Should you spare Blake, she can be found in the bottom of the Afterlife Club in Omega where she will tell you about how she is using her skills for good after you gave her a second chance.
In the Upper Market section of the Wards, a human named Conrad Verner stands atop the stairs and will attempt to get Shepard's attention. Once talking to him, you'll instigate a sidequest known as "Citadel: The Fan." Assuming you humor his wishes and check back after every main mission, you'll eventually have an option to confront him when he asks Shepard to vouch for his candidacy as the second human Spectre. If you have enough Charm or Intimidate points, you can convince him that his talents are better suited elsewhere. However, if you don't use the Charm or Intimidate options, you'll later find out that Conrad died attempting to take on a group of Turians. Assuming Conrad lived through ME1, you'll be rewarded with one of the funnier parts of Mass Effect 2 as Conrad is attempting to fill the void left by Shepard's death on the Eternity Bar in Illium.
In a café directly near Executor Pallin's office on the Presidium, an Asari named Nassana Dantius seeks out your help in regards to her sister. Should you choose to help her and complete the sidequest, you'll eventually run into Nassana in ME2 where she'll be quite surprised to see you.
Near the Human Embassy, a man named Samesh Bhatia is attempting to recover the remains of his lost wife. If you accept his quest and speak with the Alliance representative in the café, Samesh will send you an e-mail in Mass Effect 2 thanking you for helping him and his lost love.
Helping the Consort Sha'ira with her problems with the drunken General Septimus will eventually get her to offer you a reward. Regardless of which reward you accept, Sha'ira will send you an e-mail in the second game wishing you luck on your future endeavors.
After you've completed at least one main quest in Mass Effect 1, upon returning to the Citadel you'll be confronted by Westerlund News reporter Khalisah bint Sinan al-Jilani. There are numerous ways to go about the interview (The bottom right option being the most satisfying :D), but regardless of how it ends she can be found in front of the Dark Star Lounge in Mass Effect 2. Talking to her again will reflect both your actions at the end of ME1 as well as your conduct in her previous interview with her.
When you first arrive in Port Hanshan on Noveria, you'll quickly learn that you need an item called a garage pass to leave. No matter what direction you go in attempting acquire one, they'll all go through a woman named Gianna Parasini. You're eventually given a chance to either help her, sabotage her, or basically bypass her. Unless you choose the most confrontational scenario involving Administrator Anoleis (which leads to her death), she can be found outside of Liara's office in Nos Astra on Illium in ME2.
As you work your way through the mission and finish your confrontation at the Peak 15 research facility, you're given the option to either kill or spare the Rachni Queen. While the only things mentioned in Mass Effect 2 are a message delivered from the Queen and a few news broadcasts, it is likely that sparing the Rachni will have a dramatic impact on the conflict that occurs in Mass Effect 3.
The conflict on Feros doesn't have a lot of bearing on Mass Effect 2, but the actions you take on Zhu's Hope do resurface. Assuming you did all you could to help Zhu's Hope in Mass Effect 1 (kill the Alpha Varren, restore the water supply, spare the colonists) and that upon defeating the Thorian you spared Shiala, she will appear on Illium near the Baria Frontiers kiosk once again requesting your aid. After you help her for a second time, she'll thank Shepard for his/her efforts and mention the current state of the colony.
Virmire without a doubt has the most variables on Mass Effect 2. It's important that you do Virmire last seeing as how it basically sets you on a path towards the end of the game. I'm going to attempt to cover them chronologically starting with one of the more prolific moments in the game.
Upon meeting the Salarians under the command of Captain Kirrahe you learn the true nature of Saren's research. Wrex is understandably upset and it's up to Shepard to attempt to talk him down. If this is your first playthrough, it is unlikely that you will have enough Charm or Intimidate points to convince Wrex otherwise and you'll be forced to kill him. The only way around this if you lack the sufficient skill is to complete Wrex's Family Armor quest BEFORE you land on Virmire. Assuming you meet either of these two checks, you'll succeed in calming Wrex down and as a result you'll be able to see him in Mass Effect 2.
The next big thing revolves around both the fate of Captain Kirrahe and Ashley Williams/Kaiden Alenko. In order to mount a successful assault, you need to entrust the command of one of your soldiers to Kirrahe. While this choice is not so much representative of a "NO TURNING BACK" point, if you intend to save Kirrahe and his men you need to choose carefully. Obviously, the fact is that either Ashley or Kaiden die in this mission, so your choice is obviously one of preference. Whether it be for romance or any other reason, if you intend to save Captain Kirrahe, make sure you send the person you want to SAVE, with him. Upon the start of the mission, you need to disrupt the Geth communications, sabotage the fuel, and make sure that you DON'T trigger the alarms in order to save the Captain. I'll detail the final part of this in a bit.
Assuming you followed all of the instructions above; you'll eventually make your way through the cloning facility. Near the end you'll run across an Asari Scientist named Rana Thanoptis whom you can choose to either kill or let go. If you let her go, there'll be a rather humorous encounter with her in Mass Effect 2 during Okeer's recruitment mission on the planet Korlus.
At the finale of the mission, you're now given the opportunity to save one of your crew and Captain Kirrahe. Assuming you've followed the guidelines above, Kirrahe and whichever squad member you want to live should be fighting at a position back from the nuclear device. They'll radio for help and you're forced to pick between helping defend the nuke or rescuing the Salarians and your other squadmate. Make sure you choose to rescue the Salarians and you should be able to save the Captain and his squad. Of course, doing this ensures the squad mate you left to arm the nuke will perish in the blast, but it's unavoidable. Doing so will get change conversations you have with Mordin in Mass Effect 2.
Admiral Hackett will continually give you missions aboard the SSV Normandy, two of which I'd like to note. The first revolves around shutting off a rogue VI program located on Luna (the Moon). Completing this mission doesn't really offer a huge dividend in Mass Effect 2 (only a news report mentioning the facility is added), but it allows you to fully develop your character and pick a specialization class in Mass Effect 1, which is invaluable.
The next important assignment you receive from Admiral Hackett is that of stopping Geth Incursions in the Armstrong Nebula. Upon completing these, you'll have the option of using the outcome to benefit your squadmate Tali. If you talk with her after completing UNC: Geth Incursions, she'll request that you give her the data that you recovered in an attempt to complete her pilgrimage. If you agree, she'll greet you with a much warmer welcome at the beginning of Mass Effect 2.
If you habitually talk with Garrus in the bottom of the Normandy after every main mission, you'll eventually learn about a case he had while at C-Sec revolving around a Salarian named Dr. Saleon. Should you investigate this further, you'll eventually be able to track down the Doctor and either spare him or kill him. Allowing Garrus to kill Dr. Saleon changes his personality in Mass Effect 2 and shapes him into sort of a pupil of Shepard's. For instance, if you complete this, it is revealed that in between Mass Effect 1 and 2, Garrus attempted to train as a Spectre again per the Commander's example.
Finally, any romantic involvements that you pursue in Mass Effect 1 will obviously have influence on ME2. Whether it be Ashley, Kaiden, or Liara it will change the opening of ME2, the appearance of Shepard's cabin in ME2, amongst other small bits. Also, when you meet up with your love interest in the second game, you'll understandably be greeted with a warmer reception. It's also important to note that only if you romanced Liara in the first game can you continue said romance in "The Lair of the Shadow Broker" content pack for ME2.
After you finish Virmire and travel to the Citadel, you're once again stuck on a fixed path to the end of the game. As I said earlier, any business you have to take care of in terms of talking to your squad or side missions should be done BEFORE Virmire. Basically once you pass through the Mu Relay and complete the Ilos mission, you're at the end of the game. About midway through the final battle you're given a very important choice…
You're given a very important decision regarding the fate of both the Destiny Ascension and, more importantly, the Citadel Council. While neither choice is inherently wrong, it is without a doubt more beneficial to save the Council. Upon doing so, the Council will reinstate your Spectre status at the beginning of Mass Effect 2 and this will open up a lot of dialogue options that are not present if you let the council perish.
Regardless of which path you take above, you're then given the option of who the new human representative for the Council will be. The two choices you're given are Captain Anderson and Ambassador Udina. Fortunately, the save game that you'll import is recorded before this point and it doesn't matter who you choose as you'll be able to change your pick at the beginning of Mass Effect 2. On a quick note however, even though there is no official canon for the Mass Effect universe, in both the book Mass Effect: Retribution and the mini-web comic Inquisition, Donnell Udina is the human representative to the Council.
So there it is, a list of changes that will immediately change your game when you import it to Mass Effect 2. Don't treat this as a de facto guide though as I merely did it off the top of my head based off my multiple playthroughs of both games. Not only that but as I stated above, there is no true perfect Mass Effect save! The most important thing to remember about these games is that they're YOUR story and to play them how YOU want to.