Probably Mass Effect 2 given how many times I've played it, but the way Mass Effect 3 ended left such a bad aftertaste for the entire series that I have a hard time divorcing the fun I had with ME2 with the frustration I had with ME3.
@andorski: How is it impressive? It seems very much in line with expectation. It also seems like there is little to no difference with the ps4 version (judging from the screens). What is impressive is the apparent scalability demonstrated by the minimums.
Also note that R* may still cock it up, based on past history.
The game's recommended requirements matches the Witcher 3's minimum requirements as well as falling below recommended specs of most open world games that came out this past winter. So long as it's not a buggy mess R* seems to have knocked it out of the park.
The Citadel DLC for ME3 was the best use of fan service to me. It completely highlights the content you remember the most about the game - the characters - and weaves it into a story that emphasizes how Shepard's relationships with his crew was his ultimate power.