Here's my ranking:
ME2 >>> ME1=ME3
Mass Effect has always had fairly boring gameplay. The series is good for one thing: making an interesting universe and populating it with interesting characters.
By that standard, ME2 is by far the best. Every genuinely great character is introduced and properly developed in ME2. ME1 has...Wrex, maybe? Everyone else in the game is incredibly dull. Even eventual awesome ME2 characters like Garrus are weak in ME1.
ME3 cashes in on the character development in ME2, but adds nothing by itself. For example, it has Mordin, but Mordin's dialogue is weak and the only reason anyone cares about him is because he was so well-developed in ME2. Same with Thane, Miranda, Samara, The Illusive Man...hell, everyone. Previously average characters, like Jacob, become downright boring.
Outside of characters, all of the game have strengths and weaknesses. ME1's clearly the best in some important ways, but it has severe flaws that ME3 doesn't have.
I dunno. The more I think about it, the more I think that ME2 is the only truly great game in the series.