*White World (Sonic Generations) (This covers the first three classic levels)
*The ARK (Sonic Adventure 2) (This covers the "Dreamcast" era.)
*Spagonia (Sonic Unleashed) (I would of went with Eggmanland, however the ARK covers the final stage idea. Secondly it was most likely the most popular stage in Sonic Unleashed and thirdly, due to it's Sonic Generations Enhancement, we have more material to work with.)
Those are my votes, how about yours?