The thread title says it all. Anything that will play on an old Gameboy Color. 

Gargoyle's Quest was my personal favorite Gameboy game. It's kinda "Metroidvania" but not quite. I played it again for the 3DS several years ago and it still held up. It's not too long, though.

And of course, Link's Awakening is pretty amazing. I played it for the first time on 3DS and it was all I could think about for roughly 3 DayS.

Zelda: Link's Awakening DX
Zelda: Oracle of Ages
Zelda: Oracle of Seasons

Azure Dreams was a really cool title that I sunk a ton of time into, it's not exactly a GBC game it's one of the black cart games which works on the OG Gameboy but also gets correct colours when used with a GBC

Sorry if that video has odd audio, I just wanted to link some gameplay so you could see the type of game that it is. One bad thing about the game which I recall was there was certain enemies who could hit your mon with a permanent level down move, if you killed those enemies after they hit you with it you would recover your level... but if you were say lvl 15 and 75% into the level when you got hit you would go to 14 then if you killed it you would go to 15... and no exp, so later on in the game they really really hurt. The game was one of the first I played which featured rogue like procedurally generated stages.

I think I've heard good things about Metal Gear: Ghost Babel (or simply Metal Gear Solid here in the west), although I don't have any experience with it. Still, might be worth a shot.

Legend of River King
Legend of River King 2
Last Bible (Shin Megami Tensei with fantasy monsters)
Final Fantasy Legend (inspired Pokemon creator and you can tell)

Best non Hideo kojima directed MG. It stands toe toe with something Kojima would write.

Shantae is a hidden gem in the GBC library that you may want to get your hands on.

Wario Land 3. Plays quite different from Mario games and even other Wario games. You go through levels to find keys, then find the treasure chest they go to. You cannot die, only be injured in ways that look like they came from a Looney Tunes cartoon. Getting hurt in these ways is actually a huge part of the game, as they're effectively powerups. Get smooshed by a crushing enemy or obstacle, and you become a pancake, and can float down gently like paper and fit into cracks, for example. There are also numerous treasures that either affect other areas in the game (turn day to night, make an obstacle disappear, etc) or give you a new ability. In this way, even though it has levels, it plays somewhat similar to a Metroidvania. Progress as far as you can in each area, note things you couldn't do, unlock new abilities or find new ways around things and come back later to progress more. Just because you can't die doesn't mean it isn't hard, by the way. This isn't Kirby's Epic Yarn. This is a game where you fight bosses and they inflict you with statuses that in other situations might be a powerup, but in the boss' arena is incredibly inconvenient and usually knocks you out of the arena, perhaps falling a great ways or into a current too strong for you so that you lose a ton of progress. So bosses are usually quite challenging and you often can't let yourself get hit. The regular puzzle platforming challenges are often pretty tough too.

Oh while I'm talking about Metroidvanias, there's the obvious Metroid II: Return of Samus. It hasn't aged greatly, but it might be neat to see the game before it was remade, and if you like Metroid and can't play Samus Returns for some reason, it's an important part of the lore.

Pokemon is obvious, both R/B/Y and G/S/C, but an often forgotten spinoff game is Pokemon Pinball. It has a rumble pack, so it's a very unique cartridge. I played that game so much, possibly more than the actual mainline Pokemon games. It's just so addictive.