I love the Mass Effect series, but I don't think any individual member of the trilogy can be considered on its own to be among the best. ME2 has serious failings that somehow get overlooked - maybe people didn't play the first one?
ME1 has a better main story and more RPG elements. However interface/game play was very clunky, side missions meh...and Mako mining was a chore.
I always treat all 3 like 1 big game. Makes it easier. Plus each one is lesser if you hadn't played the one before it. But I always had:ME2 had a mess of a main story, uninspiring gameplay and a massively descaled world to explore. But it had excellent side-quests around its characters, which take up the bulk of the game.
ME3 completely lost track of what the story was, had dumb characterisation and stopped you from exploring the world entirely. But its gameplay is fun.
ME1 - Best story
ME2 - Best story telling
ME3 - Best gameplay. Had the best of the RPG systems and making pyro and biotic explosions is amazing