Mine is ME2, because in my opinion, it unites the best possible qualities which neither of the other two seem to possess. Like the graphics are a lot better than ME1 and not bad if compared to ME3(practically the same), the amount of conversation is just right, ME3 has too little. The mix between dialogue and action is just right, which isn't the case in ME3 which to me has too much action and too little dialogue. Not to mention that the action physics in ME3 isn't as good as that of ME2.
What about you?
What about you?