The Master System is a joy to play with... and I happen to like the minimalist Western box art! Here are some of my favorites. Note that the ones that have 16-bit counterparts, the Master System versions are actually different enough and good enough to be worth checking out, and aren't just lesser ports of the same games.
Shooters: Power Strike, R-Type, Fantasy Zone, Fantasy Zone II, Cloud Master
Platformers: Shinobi, Alex Kidd in Miracle World, Alex Kidd in Shinobi World, Ghouls 'n Ghosts, Kenseiden, Master of Darkness, Castle of Illusion starring Mickey Mouse, Land of Illusion starring Mickey Mouse, Sonic 1 & 2
Action with RPG elements: Wonder Boy in Monster Land, Wonder Boy III: The Dragon's Trap, Golvellius, Zillion
Two Player Co-op: Rambo, Time Soldiers, Double Dragon, Rampage
Light Phaser Games: Gangster Town (try it dual wielding Light Phasers in two-player mode with each one running through a Rapid Fire Adapter!)
3D Games: Maze Hunter 3D, Space Harrier 3D
Sega Cards: My Hero, Teddy Boy