Why do you think so?
Purely atmosphere wise ME2 is teh best of the series imho.
Well if you like the atmosphere, then you like it. Nothing wrong with that. I did not like it, I liked the first game, the retro 70s/80s future, and wanted to play more of that. ME2 is not more of ME, it's its own thing, going its own direction. It's fine.
ME3 returns to the ME1 atmosphere and style.
Setting atmosphere and style aside, the game mechanics that introduced in ME2, such as combat, weren't changed as such, but refined in ME3, in a good way.
There was more equipment variety in ME3 than ME2. There was something to buy in shops. Auto fish-feeder! And lots of weapons.
The story of ME2 was an in effect an interlude, while the story ME3 more or less picks up where ME1 ended.
So apart from fleshing out the universe (which is a good thing, all in all) the main story of ME2 was merely tangentially connected to the overarching plot found in ME1 and ME3.
And to be honest, I was turned off to ME2 when Shepard was killed at the start of ME2 just to be resurrected. There are other ways to start at level 1 again and it introduced more contrived plot conveniences than I feel the Mass Effect universe can bear (it's already holding a full hand in that department)
Don't get me wrong. ME2 was no DA2, far from it. I don't know what the fuck happened to DA2, probably fucking fat fag Gaider. But I didn't appreciate the atmosphere and style of ME2, to me it was trying to hard to be dark and edgy - but in a very sanitized way and the game mechanics introduced in ME2 were all refined and improved upon in ME3 anyway.
ME2 renegade wasn't as much badass dirty Harry/Plissken as it veered into being really banal evil. Stupid evil at that, not smart evil (e.g. the renegade decision in the Samara quest... my eyes rolled)