2D Sonic on Genesis, Sega CD and GBA were absolute joys to play. I could beat a level a hundred times on those games. To me Sonic is sort of like a racing game, you find lines to improve your time. The issue with the transition to 3D for me has been the camera and controlling Sonic. Enemies are little threat but all those falls into oblivion made the games often frustrating. I still enjoy them, but for me 2D Sonic ranks among the best gaming experience one can have.

If I made a Sonic game it would be the following.
1. 2D and sprite based
2. Levels would be short, two to four minutes, but have least six distinct paths
3. Story is largely meaningless, get the chaos emeralds
4. Game would actually be easy to beat, the challenge would be in time trials
5. Characters would be Sonic and ones that behave like him (Metal Sonic, Shadow and possibly Knuckles or Tails).

I had hopes Sonic 4 would be this, but they needed to really study Sonic 3 controls and design more.

2D, because for some reason, the absolute best Sonic level design I've experienced can only be found in the 2D entries. Mostly the earlier ones.