Sonic 2 is by far the most popularly
hacked Sonic game, and it has the most support available for it.
This game has some of the best hacks in the scene, including Sonic
2: Aluminium Edition, Sonic 2 Advanced Edit and JohnnyUK's ever-popular
Project Shadow.
Technical Hacks
Return to the Palace
A great hack for Sonic 2 that brings back Hidden Palace Zone.
Angel Island
A port of Sonic 3's Angel Island to Sonic 2!