Gargoyle's Quest - Fantastic early GB title from Capcom. Despite the hardware and the point in time that it released in the GB's lifespan, it's aged remarkably well.
DuckTales 2 - The first GB game was an okayish remix of the NES game, but the second game is a far, far more polished affair, I'd arguably say this bests the NES version.
Mega Man IV - Of all the Mega Man games on the GB, MMIV is the most impressive, even over V. Aside from being probably the most polished entry on the system aside from V, there's a lot of really great mechanical and aesthetic additions that really help the game shine, some of which actually found their way into the numbered console entries starting with MM7. And the game has actual cutscenes! That's insane for a GB game!
Metroid II: Return of Samus - While still a bit lacking, especially in the sound design department, Metroid II adds a whole host of conveniences and upgrades over the original game that make it less of a daunting experience compared to it's older NES brother. It almost feels like a testing grounds for some of the stuff that would go on to be included in Super Metroid, honestly.
Operation C - Despite the hardware, this is one of the most solid entries in the series, in my opinion. The level design doesn't seem as cramped as you would think for a handheld Contra entry, and it probably has the best alternative play style stages in the series this side of Super C.
Donkey Kong - One of the GOAT videogame remakes. Literal hours of fun. Not much to say about the game that hasn't already been said.
Tetris - Do I really need to explain this one?
Kirby's Dream Land 2 - Kirby's Dream Land 2 is impressive in that it manages to feel like a sequel to both the original game and Adventure: The levels are smaller and more in line with Dream Land 1, but the level ordering and copy abilities are straight out of Adventure. Great replay value too, even if finding some of the rainbow pieces are a bit annoying. Another game that holds up surprisingly well even compared to it's newer entries.
Pokemon Red/Blue/Yellow - See Tetris.
The Legend of Zelda: Link's Awakening - I'm not even much of a Zelda fan, but even I understand why this game is completely godlike. Fun items, fantastic pacing, a bunch of neat secrets, I could go on. Someone more versed in Zelda than me can probably articulate better, but as someone that's only ever enjoyed three or so Zelda games, this one is the standout to me, and the Game Boy is a better piece of hardware for hosting it.