I have my Top 5. Mind you, I haven't played every Genesis/Mega Drive title available, but there are my favorites....
1. Red Zone - The game looks fantastic. Really, the opening cinematic looks incredible, taking into consideration the limitations of the format. Also, great use of color and 3D graphics. Quite stunning.
2. Virtua Racing - As far as I know, it's the only Genesis/Mega Drive title to utilize the SVP chip, which was capable of rendering 300-500 polygons per second. The frame rate is solid and the visuals, though nowhere near "arcade perfect", are a sight to behold.
3. Wiz & Liz: The Frantic Wabbit Wescue - While it's an incredibly simple puzzle/platformer, Wiz & Liz has some the smoothest, most colorful visuals I've seen on the Genesis. Once I saw the Psygnosis logo in the night sky being reflected beautifully in the water, I was kind of awe-struck. Gorgeous game.
4. Flashback: The Quest for Identity - Still one of my favorites. Great use of color, first-rate rotoscoped animation, and the cinematics are smooth and stylish. Fantastic piece of work.
5. Outlander - I love this game. The animation may be a little choppy, but there's still a strong sense of speed and I love hitting bikers, watching said bike explode while they fly over the hood and hit the pavement in my rear-view mirror. Great fun!