I enjoyed them all, but Mass Effect 2 was my favorite.
Mass Effect 1 had the best writing in the series, but it also had gameplay that was so terrible it makes it hard to even get through the game, regardless of how good the writing was. The combat was clunky, the framerate was a nightmare, and the Mako had horrible controls and physics. Not to mention, the AI was completely crap. The friendly AI often did nothing at all, disobeyed orders, and stood in your way. The enemy AI switched between "pathetic cowards" and "zombie rushing spammers", both of which were annoying. Sure, the story was excellent, arguably the best of this generation, but its gameplay was just too terrible to redeem it to the same level as the other two games.
Mass Effect 3 had the best gameplay, even if a few RPG features were removed, but it also had the worst writing. It didn't add any interesting characters, except maybe James and Traynor, and most of the good writing was due to its building off the other games, but that was also its weakest point for ignoring points in the previous games, especially the first one, and failing to really build any more lore to what was established from the first two, except in ways that weren't all that great. Still, regardless of its problems, it still had a good story and gameplay, which is more than can be said of the first game.
Mass Effect 2 probably got the balance on story and gameplay best. The story both built on what the previous game had and added to it, and while the overarching story was the worst in the series, it still had the best characters, and it probably had the best story for what the gameplay featured. The combat was vastly improved, adding more quick slots for powers, as well as fixing the wheel so it was easier to select the ones you wanted. Battles were also better paced than ME1 or ME3. While I didn't appreciate the complete scrapping of the Mako sections, I did enjoy no longer having to deal with such awful vehicle sections (at least the DLC added a few enjoyable ones). Oh yeah, and it easily had the best end sequence in the trilogy (invading the collector base while assigning tasks to your team was excellent), and the final boss was by far my favorite battle in the whole trilogy. Yes, I didn't appreciate them making it feel a lot less like an RPG and more like a traditional third-person shooter, but that is hardly a complaint when stacked up to its strengths. Oh yeah, and it had my favorite aesthetics in the whole trilogy, and was probably the only one I really enjoyed rather than just being apathetic to.