Sim City - Dullsville is shaping up nicely, 80k+ people, now they've got a stadium.
Atomic Robo Kid - Dunno how I could miss the game back then, it's awesome and quite challenging. Good graphics, controls are spot on.
The Adventures of Batman & Robin - This is a great looking game that plays like a dream, controls are super responsive and the variety of moves is awesome, it's very satisfying to plow through enemies and the bosses are just insane. Difficulty is up there, tho, to a point where it becomes frustrating to replay a level over and over again but it's worth it.
Lords of Thunder - The Mega CD version may not be as good as the PCE port but it's fun and has a great soundtrack.
Dynamite Deka 2 - Just never gets old, love the game.
Zombie Revenge - People rarely talk about the game, probably because most don't like it but I think it's great, it was one of the first games I've played on the system. Good mindless fun.