I will suggest these games.
Final Fantasy Adventure / Legends 1,2, and 3
What old school rpg enthusiast wouldn't suggest these?
Legend of Zelda Link's Awakening
Great jump for the Zelda games in my opinion. Especially for the old gameboy.
Kirby's Dreamland
The first GB game I owned and loved. Still have it also!

Pokemon Red, Blue, and Yellow
The warped beggining of a series that brainwashed kids in to capturing big animals in a cramped tiny ball squishing the shit out of them. Then mainly only relesed them long enough to beat the crap out of each other.
Also a good twist on a collecting type of game with major rpg elements.
These are also the only games for the gameboy that I've printed out covers for so far.
Tetris and Tetris 2
I don't know how many hours I've spent wasting a few minuits here and there just to pass the time with these games.
All of the Mega Man Games
Great platform games right here.
Battletoads
An extremely hard yet good platform game.
These are just a few that I have picked out of my personal collection... I'm sure there will be some that dissagree but I think they are all good games.