I think, pound for pound, all the characters in ME2 are better than ME1. I kind of hated everyone who wasn't Wrex or Tali in that game. The problem is that there was probably too many of them.
In ME1, Wrex factored pretty majorly in to the actual story. Tali's civilization was at war with one of the antagonists in the game. Garrus wanted to take Saren down because he was a cop with a strong sense of justice and was being stonewalled, Liara's mother was a major villain, and then there were the two humans.
In ME2, it's Shepard's Space Adventure. Everyone had motivations, but they were basically just "This is bad, we should stop it!" They all had backstory and were mostly interesting characters, but I just never bought their involvement in the actual main story.
ME2 felt more like a set of vignettes connected by Shepard, which may have been the actual intention. I just liked how cohesive the first game felt.