I just bought a Genesis, and, while I am no expert on the subject (yet)...
Vectorman: Great shooter, difficult if you are used to shooters like Gunstar Heroes... Wherein fights are masses of enemies that one shot can kill all of them, in Vectorman one fight can cripple you. It's pretty too. Reminds me a little bit of Balluz, and I HATE that game, but it actually looks really pretty. I have yet to see Vectorman2, and can't wait.
Rocket Knight Adventures: This game has a lot of different 'modes' I guess you could call it, of gameplay. One minute you're a sidescrolling, shooter-style/platforming 'possum, the next, you're flying through the air in an effective and nifty traditional shooter, the next you're fighting creative bosses that take a few tries to get (the railroad boss... ugh), the next you're swimming around avoiding mines. Very creative level designs, from a level where the water level goes up and down, and you have to swim around spikes... very hard, to a level where the reflection in the hot lava is the only way to know where you're jumping... Great game, and pretty. Better than Megaman....
Ultimate Mortal Kombat 3: This game incorporates the fighting style present within MK3, but adds a butt-load of new characters and new moves. This game really kicks ass when using the three-button controller, because you have to know what you're doing to shoot fireballs, or things like that. My favorite fighting game.
Sonics: Of course. All the Sonics are amazing, and Sega even had the brilliant idea to add Knuckles to pre-existing games. I was so angry when I saw in the first level of Sonic 3 three blocks that I couldn't break through. But with Knuckles, it opens up whole new areas of the levels to explore, thus adding so much more to a game you've already played through. Amazing.
That's all I'll list for now, but I can't wait for the brand new Genesis game to be released, Beggar Prince. I don't care if I don't like RPGs, I want to give them support so that they will continue making brand new games for the best console ever!