First was the most unique, but in some aspects it really did not age well.
ME1 felt more influenced by 80's sci fi, later games were more "modern".
Best main story, best ambiance, sense of mystery, lore, music.
Character writing? Not so good, more exposition dumps. Really poor progression system, itemization/weapons ( though shotguns/sniper rifles sounded awesom), economy and exploring barren planets/whole second half was just tedious.
I'd kill for remake with Andromeda gameplay in it.
Yeah, it was the overall atmosphere that made ME1 good. It really felt like a grand space opera, and that feeling, at least to me, was greatly diminished in ME 2 and almost entirely absent in ME 3.
I liked the companion quests in ME 2 and felt like the main story was on par with ME 1, but almost everything else was a step backwards imo. The level design was much more linear, and there was far too little variety in the enemies and combat in general.
I thought ME 3 had the worst story by far, but I enjoyed the combat more than in ME 2, and I think it generally gets a worse rap than it deserves. That said, it was a somewhat disappointing end to the trilogy.