Too many to list! At least folks have started out with a nice bunch...
Here are some that you should consider, but only if the blurb I wrote appeals to you...
Gaiares —only if you are a hardcore shootemup fan.
Valis III — To see a great* game, but to see how the TG-CD is even better. Action-platformer. * Most folks wouldn't call it great. I like it, though.
Phantasy Star II — even if this genre—ARPG/RPG—normally doesn't appeal to you, you must play Phantasy Star II.
Herzog Zwei — one of the first Genesis games I had a blast playing with a friend (some battles were ridiculously awesome). Early real-time-strategy...might not be appealing to you.