Green Hill is a stage in Sonic Generations, being the 1st stage in the game, and in the Classic era.
ClassicEditClassic Sonic's rendition is a remaster of the original zone. In the console version about 3/4 of the level is entirely new. In the handheld version, the stage is identical to the original (Act 1), except in 2.5D. It has corkscrews, spring, swingsets, etc.
Modern's stage is new, completely. It has rails, and a new section in a cave where a Mega Chopper tries to eat Sonic. The stage ends with free-running.
In the handheld version, the stage is similar to the Sonic Adventure 2 version. Instead of a giant Chopper, you destroy a giant totem pole.