I would disagree about SA2 (especially the music, though parts of the game were annoying).
Call me whatever you want, lol, but in a way, some parts of Sonic Adventure were revolutionary (graphics, running from whales, Windy Valley, anything that didn't suck). At the time, there was never such a game known to mankind!
I remember fully playing Sonic 3 after I played Sonic Adventure (I played adventure in '99; I played Sonic 3 once a long time ago but didn't remember it because it was so brief). I enjoyed Sonic 3 a hell of a lot, even after SA.
Sonic Team's biggest mistake: Big the Cat. WTF? Plus they have too many characters... Silver, Cream, Cheese, Vanilla, Pastrami, Frozen Yogurt...