Personally, I prefer Mass Effect 1 for building the overarching plot, characters, and brilliant universe. Implementing things like the new dialogue wheel, seamlessly integrating speech checks into conversations to make it more personal, and having player choices have lasting impact were huge! When you get the unveil of the reapers and you meet Sovereign, when you meet Vigil on Ilos, exploring the Citadel for the first time, I remember this game from start to finish.
Mass Effect 2 will probably win this. I can see why too. It nailed it as a stellar sequel to a great game. ME2 improved the shooting and trimmed the fat ME1 had. ME2 also improved the dialog system as well as adding the paragon/renegade LT/RT system for more branching conversations.
I didn't like losing armor configurations to one single suit, I like number crunching and ME2 didn't deliver on that front. Even regarding criticisms however, I can still critique ME1 more than ME2. ME2 is more polished. ME2 is the game that made ME3 such a blockbuster event.
Conclusion: Objectively, ME2. Personally, ME1.