Sonic Adventure 2 Battle (SA2 felt a bit different) or SA1 (DX felt a bit different) for the 3D releases, Sonic Battle & Sonic the Hedgehog 2 for the 2D releases.
I'm going to out and say it: all 3D Sonic games after Heroes were terrible IMO. They didn't do the things that a typical Sonic game SHOULD have done. Sonic 2006 was an embarrassment; it's nearly unplayable for me. Colors was a step up but the boost mechanic was iffy, same with Generations. The prime of 3D Sonic was in it's beginnings -- the Adventure series.
For the 2D releases, I love them all, every last one of them. I count Sonic 3D Blast in the 2D games though many will argue it's 3D in some capacity. Sonic 3 & Knuckles was great to be sure, but Sonic the Hedgehog 2 is the game that holds the best memories (almost beat it all in one sitting!) for me.