Man, looking through I actually don't have as much fondness for the Genesis as I thought I did. Most games I played on it where multi-platform where I preferred the SNES version and where I would have trouble narrowing down my SNES list, even filling a Genesis one is kinda hard.
-Sonic 2
-Sonic 3 & Knuckles
-Sonic
Obvious choices out of the way, these are the reason I bought a Genesis (though used from Gamestop for cheap after the systems hayday) in the first place. I hold the slightly controversial ranking in which I prefer Sonic 2 over Sonic 3, though this may be because I never completed a full play-through until a few years back where I had played 1 & 2 fully as a kid.
- Gunstar Heroes
One of the few true things to hold over the SNES in playground debates. I didn't play it until later, but it's amazing in gameplay and music while being very unique. Seven Force is in my top 10 bosses ever for sure.
- Rocket Knight Adventures
Really unique gameplay gimmick and excellent music. Stands above the rest of the non-Sonic mascot platformers.
- Castlevania: Bloodlines
Stands toe-to-toe with the SNES' Castlevania offerings. Not up to 4's standard, but perhaps better than Dracula X.
- Aladdin
The SNES and Genesis version are different enough to be considered their own game. Really tight platforming.
This is really where my list proper ends. There are a lot of games I enjoyed on it as a kid (Vectorman 1 & 2, Mickey & Donald: World of Illusion, Golden Axe, etc.) but they have failed to remain as prominent in my tastes as the above games or their SNES cousins.