1) Sonic the Hedgehog 3 and Knuckles. This combined game (really a full game that was split into two cartridges) of Sonic 3 and Sonic and Knuckles is to me the single greatest Sonic game to date. I did a full review of this before, so I won't go into this too much here, but with some of the best graphics the Genesis ever had, three different playable characters with very different game play, and a really strong narrative for a 2-D platformer at the time, this remains to this day my favorite game of all time.
So there you have it, my ten favorite games? Don't agree? Well, as always, make your own damn list then.