Wikipedia says the Sega Genesis library has a total of 713 games. Nowadays with storage not being a problem and emulation you can have the entire library. But lets say you didn't want all those sports titles and educational stuff. Lets list what the essential no BS library of genesis games should be. Doesn't matter the region either. I'll start. You can add onto mine or create your own.
Golden Axe 1,2,3
Sonic The Hedgehog 1,2,3
Sonic and Knuckles
Streets of Rage 1,2,3
Castlevania Bloodlines
Gunstar Heroes
Chakan the forever man
Vectorman 1,2
eternal Champions
Mickey Mouse castle of Illusion
Mickey Mouse world of illusion
Aladdin
Desert Strike
Urban Strike
Beyond Oasis
Adventures of Batman and Robin
Street fighter 2 CE
Comix Zone
Dick Tracy
Earthworm Jim 1,2
Moonwalker
Road Rash 1,2,3
Revenge of Shinobi
Shadow Dancer
Shinobi III