What's your preferred era of Genesis titles? The earlier black box/white grid look, or the red box with the new logo? It's a close call, but I'd have to say that if I could only play games with one type of box or the other, I'd have to go with the classic black box games.
Red. I can see the draw of the black-box era for some hardcore Sega fans since that's when they did most of their arcade ports (including some excellent ports of Capcom arcade games), but the red-box era has: