Hellfire Saga

This is, without a doubt, one of the most interesting rom hacks I've ever played. As an avid fan of the old horror-esque platformers like Castlevania and Ghosts'n'Goblins, as well as a fan of Sonic, seeing it meshed together is amazing. The music is fantastic, some of the stages have neat little tricks (Like in Forest of Decay Act 3), and the HP system is very well implemented.

Sonic 3 has always been sort of lacking in the hacking department. With two very interesting hacks being released in the past year (this and S3K battle Race), maybe Sonic 3 can start getting the same level of hacking that it's two older brothers have.
I remember playing this at either last year's SAGE or SHC, and felt it was one of the most difficult Sonic hacks I ever played. Playing it this year I am glad the difficulty has been nerfed a great deal. I also love the stage layout of this build more than the last one. Speaking of stages, without giving too much away, it would appear as if the stages, at least in Forest of Decay Zone, play mind games with you.