2D: I liked almost all of the levels in sonic 3&K but if I was to pick one it would be Hydrocity Zone as a level that dates back to my child hood and is still fun today. Honorable Mention would be Lava Reef and Mushroom Hill beening that I havent played them as much as Hydrocity. Heck the whole game would make me buy the sound track.
3D: I had a hard time with this one but I would have to go with Sonic Colors, Planet Wisp. It was a pretty level with great music. The intro alone got me hooked with the grass field converting to an industrial theme. Honorable Mentions would be the Sonic Generations Seaside Hill, Sky Sanctuary and Sonic Adventure 2's Pyramid Cave.