Agree on Dragon Trap -- one of the very few games I kept on the real SMS...
Other suggestions:
* Golvellius (Sega's answer to Zelda 2 -- best gfx on the SMS)
* Bram Stoker's Dracula (surprisingly good adventure)
* Vampire (different game to the one above, this one's more like Castlevania, and nearly as good as Konami's classic! Truly -- this is one of the best "unknown" games I've ever found!)
* Asterix (solid platformer)
* Gain Ground (time travellin' shooter, similar to Smash TV)
* Penguin Land (Mr. Driller with an egg you have to save)
* Ax Battler [GG] (Golden Axe meets Ys)
* Golden Axe Warrior (Golden Axe meets... Zelda! Really good, this one)
* Jurassic Park (varied levels, very surprising, worth trying)
* Mickey Mouse - Castle/Legend/Land of Illusion (very impressive, almost as good as the Mega Drive versions!)
* Shoniobi, Sonic, Fantasy Zone, Wonderboy 1/2/3 -- all classics
There are plenty more goodies... experiment!