Of all the Sonic games I've played the sidescrollers have easily been the best. Sonic 1 is okay-ish, Sonic 2 is great, and Sonic 3 is amazing. Generations has amazing sidescroller sections but is just okay otherwise. Colors is pretty bad, too gimmicky and it feels untight, not sure where the praise comes from. Lost World was okay-ish, nothing more and nothing less. Didn't grow up with the Adventure games so what little I've tried has been absolutely horrendous, ugly animations, broken camera, glitchy as heck, terrible controls, bad voice acting, it seems like whenever I play one of the early 3D platformers that people always praise they're always terrible, the same goes for Super Mario 64 and Banjo Kazooie.
Sonic is really one of the most inconsistent video game franchises out there, and if it hadn't been for Generations I'd probably have said it should have stopped long ago.