The greatest Sonic hacks I know of are these:
Sonic Megamix. (99% complete and the full version will blow our minds)
Amy Rose in Sonic 1
Amy Rose in Sonic 2 ( the only 100% complete character hacks I've ever seen. No missing sprites)
Sonic 2 Heroes (not 100% complete but the gameplay is incredible!)
Sonic Quick Winter Adventures (kinda bad but I like the winter setting and the last boss is quite unexpected and challenging)
Sonic 2 XL (nice gameplay and hilarious)
Sonic 3 Customizable (you get to change some things to your preference and it has Knuckles and Tails, Amy, and Amy and Tails without replacing anyone!)
Sonic 1 Remastered (incredible features)
Sonic 3 and Knuckles: The Challenges (mission styled and unbelievably complete with the most unexpected abilities you will ever see in a Sonic 3 hack)
Robotnik's Revenge (nothing but bosses from Sonic 1 And Sonic 2)
Sonic 1 and 2 (Both Sonic 1's levels and bosses ported and unchanged to Sonic 2, and still has all of Sonic 2's levels and bosses, a perfect combination of two games)
Super Sonic in Sonic 3D (a hack of Sonic 3D with Super Sonic and no Flickies)
Sonic Pixel Perfect (the most beautiful and wonderful graphics ever made)
And those are the best Sonic hacks I've ever seen.
Sonic Megamix. (99% complete and the full version will blow our minds)
Amy Rose in Sonic 1
Amy Rose in Sonic 2 ( the only 100% complete character hacks I've ever seen. No missing sprites)
Sonic 2 Heroes (not 100% complete but the gameplay is incredible!)
Sonic Quick Winter Adventures (kinda bad but I like the winter setting and the last boss is quite unexpected and challenging)
Sonic 2 XL (nice gameplay and hilarious)
Sonic 3 Customizable (you get to change some things to your preference and it has Knuckles and Tails, Amy, and Amy and Tails without replacing anyone!)
Sonic 1 Remastered (incredible features)
Sonic 3 and Knuckles: The Challenges (mission styled and unbelievably complete with the most unexpected abilities you will ever see in a Sonic 3 hack)
Robotnik's Revenge (nothing but bosses from Sonic 1 And Sonic 2)
Sonic 1 and 2 (Both Sonic 1's levels and bosses ported and unchanged to Sonic 2, and still has all of Sonic 2's levels and bosses, a perfect combination of two games)
Super Sonic in Sonic 3D (a hack of Sonic 3D with Super Sonic and no Flickies)
Sonic Pixel Perfect (the most beautiful and wonderful graphics ever made)
And those are the best Sonic hacks I've ever seen.