Sonic the Hedeghog: The original and first level with colorful landscapes made this game exciting to play and enjoy, features your act 1, 2, and 3
Sonic Adventure 2: If you get a "A" rank on all 180 missions in SA2 then you unlock the special stage of Green Hill Zone, playable with other characters like knuckles and Shadow.
Sonic generations: For the 20th anniversary of Sonic in 2011 SEGA made a game that had all of their best stages into a game. First Level and has a 2D and 3D sections as well.
You found out that Dr. Eggman or Dr. Robotnik was enslaving the Flicky animals to be turned into robots for his own evil purposes. You go through green hill to stop him.
After you beat him he escapes away and you chase him here in Marble Zone. You eventually catch up to him but still continue on chasing him.
You then find Eggman here and beat all though this time he escapes as well so you continue to chase him so you could stop him.