I'm excited for Sonic 4. I wouldn't mind Tails and Knuckles being playable, but no-one else. They added variety to the gameplay and could get through the stages in different ways.
Also, here are my top 5 2D and 3D.
Top 5 2D:
1. Death Egg (Sonic and Knuckles)
2. Stardust Speedway (Sonic CD)
3. Ice Cap (Sonic 3)
4. Lava Reef/Hidden Palace (Sonic and Knuckles)
5. Flying Battery (Sonic and Knuckles)
Top 5 3D:
1. Levitated Ruin (Sonic and the Secret Rings)
2. Egg Fleet (Sonic Heroes)
3. Final Rush (Sonic Adventure 2)
4. Green Forest (Sonic Adventure 2)
5. Sky Deck (Sonic Adventure)
Levitated Ruin gets the top spot due to 1: a sentimental reason, 2: the music being fantastic and 3: the gargoyle trail chases are wonderful. There's something about the high altitude stages in the 3D ones where one mess up means a long fall to your death that I love. Yeah, they're really annoying, but doing a perfect run on them is something that just feels awesome.