Mordin.
I liked Mr. House quite a bit, but New Vegas was about exploring the wasteland, not so much the main story which was a bit short, and so Mr. House didn't have much dialogue or backstory (A rich smart man is a rich smart man, what else is there to him?). Also, I liked John Henry Eden better.
I also would have picked Victor from New Vegas, but he's a bit character that's barely in the game.
So, I'd have to go with Mordin. Perhaps the best character out of ME2, and that game was full of great characters.