SonicMegaDrive's list is pretty damn keen.
Except I'd throw Super Hydlide in the "Not for everybody" pile. Sure it's a bit hard to get into at first but there's a lot of interesting ideas and music is great.
Anyway...
Alien Soldier- Required play, Treasure game which emphasizes boss fights and speed. Rather tough but an awesome pick up and play title.
Decap Attack- Disturbing and gross US release of Psycho Fox. It's a fairly mediocre platformer and the awful changed art is just painful.
D&D: Warriors of the Eternal Sun- Good yet rather short AD&D based RPG.
Dragon's Fury- Genesis version of Devil's Crush for the TG16 I do believe. Also an inferior version.
Dynamite Headdy- Required action-platformer by Treasure, among the best the Genesis has to offer.
Forgotten Worlds- Pretty decent port of the arcade game, though the continue system is odd(you can continue as long as one person is alive in a two-player game)
Gaiares- Overrated shooter with some nice bosses and a neat weapon system, shame everything else is dull and lousy.
Hellfire- Worthwhile shooter. Unlike some you actually have to move around and find the right angle to attack bosses from, neat idea.
Mercs- Not as good as the arcade version(but still a nice port) but there is a Genesis exclusive mode that's pretty damn hard.
MUSHA- Not the best Genesis shooter but it hits all of the points well and it's fun to play.
Mystic Defender- Older action-platformer. The final boss is
if you're into that sort of thing..other than that very skippable.
Ranger X- Another Required game. Awesome in every way.
Red Zone- Kind of meh but the game is a showcase for special effects on the Genesis. Tons of scaling and rotation effects, worth it for that at least.
Ristar- Required platformer. Slow-paced and a wonderful ride all around.
Sagaia- I believe this is simply Darius 2 re-named. I couldn't tell you much aside from that because I've only played the arcade release.
The Lion King- Good platformer though by the time it came out I was tired of all the Disney/Virgin platformers. Though it plays well the concept as a whole was so tiring I quit when I got stuck towards the end.
X-Men 2: The Clone Wars- Better than the first game but it's a pretty frustrating title all around.