I had Sonic 3 which plugged into Sonic and Knuckles, I remember playing the game to death to unlock all the emeralds which gave all the characters super or hyper powers depending if you had them all.
Doomsday zone, the boss specifically, was absolutely solid as I remember with its gravity inverted sections and stretched out save points. Was brilliant to complete it though.
Also the level where Knuckles realises Robotnik is the evil guy and not Sonic is brilliant, how I loved those games!!