For the record, Sonic Heroes would be on this list if it had a broader stage variety. The formation aspect, as fresh as it was back then, wasn't explored nearly enough, either.
3) Sonic Advance 3. The Advance series, other than this game, didn't do much to satisfy me, and maybe that's my fault, but Sonic in particular just seems really slow, especially in Advance 1. Slow and slippery is a bad combination and Advance 3 does the most to rectify this. None of the other Advance titles are as fluid as the 3rd. (Advance 2 would be close if it didn't have those dumb forever-running boss fights).
The tag team aspect again comes into play, with every character having a different moveset based on their partner, which ups playability. Example: I didn't even know you could use Amy's hammer to get more out of the springs until a couple of years ago, and that's helped a fair bit with my time trials in Ocean Base.
2) Sonic Colors. First game I poured a decent amount of time into since Secret Rings, and boy am I glad I did. The controls are sharp, the writing is pretty quality and not too serious, and the wisps are a decent gameplay mechanic that don't feel too intrusive on the experience as a whole.
Also, Planet Wisp is the most beautiful stage yet, complete with bass guitar and speeding through the levels with that in the background feels so good.
1) Sonic Rush Adventure. Sonic Rush was the game that ended my personal streak of "Another Sonic game? Eh, it's average." From the reveal trailer I was hooked. A viable rival character with believable ability? 2.5D stages, with boss fights rendered in 3D? Hideki Naganuma? Count me in. I played it, and it was hard. It was engaging. It killed my thumbs with the fight against Blaze. And it reignited my interest in Sonic games.
And then I played Sonic Rush Adventure.
Almost everything that was great about Rush was expanded upon, and the prospect of sea travel was added, making the adventure that much more legitimate because you're actually traveling to these areas like you would in Adventure Fields... while still playing Rush. It's amazing. It's amazing, but the first Rush does have slightly better music after all. Except for Deep Core. Never forget Deep Core.