I answered the survey in terms of popularity (Sonic 2), but gameplay-wise #3 was where they hit peak potential. 3&Knuckles is still the best in the series, unfortunately.

For me, Sonic Adventure 2.

The only Sonic game around my time that I genuinely liked.

Advance is nice, racing transformed was surprisingly pretty fun for me. Played it on vita, so I played it in small bursts every now and then and it seem to work really well like that.

Sonic 3 (Genesis 32X Version)

SA2 is my favorite, I have a load of nostalgia with that game, so maybe that's what keeping me from being too judgemental with it, but I always thought that the game had such an incredible sense of personality that it really doesn't feel like any other game that I've ever played, the game knew what it waas and even though it was far from perfect, it conveyed a very strong feeling of self-confidence that may or may not come as a side-effect to Sonic's natural edgy-ness. Also, the soundtrack is fucking legendary, seriously, easily one of the best videogame soundtracks of all time.

It peaked at Sonic and Knuckles. With Sonic 3 and Knuckles being the apogee of it all.

The first two adventure games were good, because of the 3D novelty, but contrary to the earlier 2D titles, they haven't aged very well. The same can be said of most titles that came after that as well.