My brother had a Genesis and a pile of games. I always thought Vectorman (1+2) was super cool, and Comix Zone, and the Streets of Rage games. He also had Contra Hard Corps and Doom Troopers which were awesome. The thing we played the most was UMKIII, though.

I'm not familiar with the master system, but the Genesis has a fair number of good games. I think it unfortunately doesn't hold up as well as it's competitor the SNES though, because Sega went hard on licensed properties and sports titles, and a lot of those games just aren't particularly relevant these days, and there were a lot of cross platform games that were just better on the SNES.

But a few that I think are worth mentioning:

• The adventures of Batman and Robin
• Contra hard corps
• gunstar heroes
• vectorman 1 & 2
• Ghostbusters
• streets of rage 2
• comix zone
• Toejam and Earl 1 & 2 (the sequel is a very different game)
• zombies ate my neighbors
• road rash 2
• earthworm Jim 1 & 2
• Castlevania bloodlines
• Shinobi III
• Rocket Knight adventures
• Mickey mouse Castle of illusion