Sonic 2 Retro Remix (Sega Genesis, 2011)

Okay, so this one of the most impressive Sonic hacks I've ever seen. The stage structure's been changed to one similar to Sonic Advance 3, each zone is a mini stage using special rings as a transport to each act.

On top of that, each act has 7 goals, completing one gets you a Chaos Emerald, which you use to open up new zones and bonus levels.

They've also added a lot of the moves from newer sonic games, air dash, homing strike, and that thing where you go a little bit higher after a spring bounce.

Also the ROM says I have to post this link or I'll get yelled at. Makes sense. http://info.sonicretro.org/Sonic_2_Retro_Remix