Sonic the Hedgehog 4

[10,000 Volt Ghost]

The top of Sonic & Knuckles opens up so you can play Sonic 1, Sonic 2 or Sonic 3 with knuckles. If you play Sonic 3 with S&K in it you basically play through all of Sonic 3 then it continues through to Sonic & Knuckles. If you get all of the Chaos emeralds you can then get the Hyper Chaos emeralds which makes you that much more of a B.A. Also there's a save feature.