All of these. ME2 is a really good game but they kicked the atmosphere to the curb.
ME1 to ME2 is basically Alien to Aliens though. The quality doesn't drop, but it flipped the tone and style of the first game on its head.
It goes from epic space opera in the fantastical, 70's/80's like, bright and clean Sci Fi, very much in the vein of 2001 a Space Odyssey and Star Trek, which ME2 changed to character driven, darker, seedier and more intimate short stories in the style of more cynical, almost grubby and relatively grounded 80's/90's stuff like Blade Runner and Farscape.
Personally I prefer the later, but I understand why people prefer the former. ME1 or 2 is just picking what flavour of awesome you like.
Now ME3, that was a dramatic drop in quality.
It Just pissed away all semblance of a coherent universe and internal logic, stripped out most of the narrative and character driven writing that made the first 2 games great, and replaced it with space magic hand holding toss, and replaced the rpg, personalising choices with more shooty shooty bang bang. Tuchanka, Rannoch and the Citadel DLC aside, ME3 was an at best mediocre shooter, but more often than not boring, annoying and massively disappointing.