Gave it some more thought.
Samara's my favourite ME2 character, but, from a story perspective, I get the feeling that she's not going to play more than a minor role, and that kind of works. I mean, as much as I *heart* her, I seriously can't see her doing much more than convincing the asari to move their fleets to protect Earth, and she doesn't need to be a party member to do that.
So, not counting Liara, who's probably a given to be a party member at this point, the ME2 character who I think is most important to keep around, purely because it helps drive the story, is Miranda. Love her or hate her (I'm in the 'meh' camp), they basically set her up to have a damn good storyline in ME3, what with her father, Cerberus, TIM and all that unresolved goodness. I always got the feeling that Bioware were setting her up to be a returning party member who does important things in ME3. I didn't get that feeling about anyone else, to be honest.