Damn it's been a while. Outside of Tetris, which is the undisputed greatest Game Boy game ever, I would say pick up Super Mario Land 2: 6 Golden Coins. It's mechanics are like SMW but the levels are all very unique, and the final boss is someone making their first appearance who went on to become big in his own right.
Pick up Castlevania Adventure and Castlevania II. It has annoying early Castlevania mechanics but is really a good game overall.
Rubang will throw a conniption fit if someone doesn't mention the Legend of Zelda: Links Awakening, so get that too. You can pick up Mario's Picross, but it would probably be cheaper and better to just pick up Picross DS if you have a DS.
I'm pretty sure the Mega Man games were good on the Gameboy.
Final Fantasy Adventure was supposed to be pretty decent.
Outside of those, I'm spent. I used most of my Gameboy time on just Tetris, Castlevania Adventure and Super Mario Land 2.
Tag: Became a freaking mod and a complete douche, coincidentally, at the same time.
