I'd probably end up choosing Sonic&Knuckles although Sonic 2 comes really close. Sonic 2 was the first Mega Drive game I ever played, after all, and I enjoyed its breezy, fun atmosphere and nice level design and memorable music quite a bit.

S&K, however, takes everything a bit further with more polished gameplay and graphics, each level having its own distinctive gimmick (yes, I dreaded those ghosts in the pyramid level, brr!), two different stories which affect the levels themselves and offer a cohesive narrative arc, and the music which is at least on par (perhaps even surpassing) S2's OST. And we finally get to explore Death Egg in detail! The final and penultimate bosses were quite satisfying in both games, too.