I prefer Mass Effect 1, but I think ME2 was a great game as well. They are pretty different games, though. Mass Effect 1 was an RPG with shooting, 2 and 3 were a shooter with RPG mechanics. I'd like them to go back towards the 1 style with the new series, but that has almost no chance of happening.
This is basically me. ME1 is my favorite game ever. When ME2 came out, disappointed didn't begin to cover it. Basically because...
What annoys me most about Mass Effect 2 is that nothing happens in it. It makes up a bunch of new problems and then solves them. The plot as of the first game does not advance at all.
And thermal clips.
This. The whole thong felt more like a side story than a main entry. Then there was the whole sole survivor/Cerberus thing that I still can't quite get over. The whole thing reeked of Bioware suddenly realizing that despite promising a trilogy, they actually had little interest in making one. It didn't help that I could also tell that the nature of the suicide mission meant that it was unlikely any of the ME2 squad members would have a major impact in ME3. I still actually find it hilarious how the ME3 squad is basically the ME1 squad, but with James in place of the dead human, Javik in place of Wrex and then EDI added on.
Regardless, on its own outside of the context of being a middle part of a trilogy which it fails at, it's a great game. I've certainly played and beaten it way more than most games.