Mass Effect 1, because it felt much more epic than the second one. It had more of a sense of exploring the galaxy, discovering interesting people and places, and plus, the story was miles better than the second one. Also, I can't think of a single person who would take loading screens over elevator rides (at least they made the world, even the Normandy, feel big, and the funny conversations your party had made them worth it). Plus, the second lacked major, holy-shit, jaw-droppingly awesome moments, like the Battle of the Citadel. Hell, the whole last mission of Mass Effect 1 was amazing.) Saren is a way better antagonist than the Collectors. Also, the leveling system, and it had less obvious black-or-white moral choices than the second one (although the action choices were pretty cool.)
Don't get me wrong, I'm not ripping on the second one at all. It is still amazing and one of my favorite games ever.
-Better combat
-Better graphics (slightly better animation)
-Way better character development
-WAYYYYY better DLC (especially Shadow Broker)
-Funnier
-Better paced