Sonic 3: Doomsday

You can't play as Knuckles or Tails in Sonic and Knuckles. You're essentially getting the first half of the game with Sonic 3, the last half of theGame is huge for a 2D platformer. Each individual level is so much larger than any of them. Tales gets the short end of the stick to some extent because all it is is just Tales except there's some birds flying around him. I wonder if that was how they inspired Cream and Cheese from Sonic Advance 2.