i just finished my first play through today...and if we're going with the themes of making critical decisions and how they effect the future...from an RP perspective, I'd say Legion is the *most* important character in your squad, in terms of story.
In my first play through I recruited all members and achieved their side missions. I didn't really have a preferred order, so I tended to recruit the members in geographical order. I recruited Tali last, did some side missions in geth space, and then went to her trial.
I know that this is the spoiler forum, but some massive spoilers follow so i'll warn you anyway:
1. Talk to the admirals and they reveal the quarian race is contemplating waging full-scalle war with the geth. Some oppose this and others are for it. Tali's father was apparently an advocate for waging war and was developing the weapons.
2. If you go the paragon route and warn the tribunal not to war with the geth, in the Cerberus mission summary it indicates that shep may have saved the quarians from almost certain annihilation by warning against war. Summary goes on to mention that this will in turn allow the quarians to use their unmatched engineering skills to help against the reapers.
I then did the IFF mission, and was surprised to see Legion there. Now, in terms of an RP perspective, even if you were the Devourer of Geth in ME1, it's quite clear Legion is very, very different. He also saved your ass from husks...which further reinforces the newly-revealed confirmation on the derelict reaper that the husks are not geth tech, but rather reaper tech. This is pretty subtle, but it greatly alters the entire backdrop of ME1: in the midst of obliterating all those geth in ME1, they were usually with husks. But now you know that there's obviously a clear distinction between husks and geth. The geth didn't create the husks. Not only did they not create the husks, but there appears to be at least 1 geth that kills husks. which led my character to question why this geth not only has speech but also opposes the husks and apparently the reapers.
And, put in the Normandy's AI section, it was probably the most secure and fast way to render Legion harmless if he were dangerous. So you activate Legion and you learn *a lot* of interesting, important facts, that alters your perception of ME1.
1. Many geth were seduced by Sovereign and those literally branded heretics. Legion even quotes verbatim what Sovereign tells the player in ME1, about how the reapers created the mass relays and the citadel to compel galactic civilizations to develop along paths that the reapers desired. When approached by sovereign, most geth rejected his advances because they deemed it to be illogical to have their decision processes taken from them in terms of having their own path of development. Heretical geth sacrificed this ability to make decisions about their path of development in exchange for adopting sovereign's methods and goals.
2. Even the geth encountered in ME is not the "true form" of geth. The "true geth" are just millions or billions of software platforms that are networked together. They don't have physical bodies. The heretical geth are all software programs downloaded onto "mobile platforms" to do sovereign's bidding. Even Legion (as he reveals in subsequent convos) is the only "mobile platform" of the true geth operating in front of the perseus veil..the rest of the geth still behind the perseus veil.
3. Performing Legion's side quest also throws a new light on ME1. You can effectively decide whether to erase all the geth aiding sovereign (and implicitly, all geth that were of the same type that you encountered in ME1) or reprogram them to join Legion's true geth. In any case, the heretical geth appears to be over. In the cerberus mission summary, if you chose the paragon route of reprogramming rather than outright destruction, the summary will say while the heretical geth are now eliminated, their numbers and resources are now added to the true geth, making the true geth significantly stronger than before. This makes it ambiguous if it's good or bad.
Conclusion: It's clear that based on Tali's sidequest and Legion's sidequest, the decisions you make will somehow determine whether the quarians and the geth wage war, and if they do, which side will win. In my RP my character is trying to rally all the sapient/sentient races of the galaxy against the reapers. My shep saved the council in the interest of preserving strength and stability in the face of the impending crisis. My shep saved the rachni queen in ME1, which yielded a promise from an asari on illium that the rachni were rebuilding and would ally with me against he reapers. Shep preserved the partially-discovered cure for the krogan genophage so that the krogran would have something to unite them and have hope for the future (cerberus summary called the krogan invaluable shock troops against the reapers). And here my Shep advised the quarians not to go to war and my Shep determined that the illogical experiences of the heretical geth would be incorporated into the true geth, mathematically proving that the reapers are a threat to be met.
If you continue talking with Legion, he tries to show access the FTL communications to let Shep hear a recording. EDI lets the communique go through, and she is greatly humbled by the experience of connecting to a hive mind that is so vast it is the size of a galactic arm (in EDI's terms). My shep wanted such a formidable race to oppose the reapers...and not be wasted in what was relatively a minor bloodfued with the quarians.