Recommendable GB games I've played or re-played in the last couple years:
Balloon Kid: A Balloon Fight sequel with scrolling levels and bosses.
Castlevania II: Very good action platformer, and the only GB Castlevania I could call essential. As much as I like CV Adventure, I can't recommend it. CV Legends is bad.
Donkey Kong: Probably the best release for original Game Boy. I say that even as someone that doesn't super dig on puzzle action games.
DuckTales 1-2: Basically the NES games, but with redesigned sprites and levels to fit the GB resolution.
Gargoyle's Quest: I don't actually feel like the RPG "wrapper" around the levels is necessary (drags things out a bit), but a well done game when you're in the action. Unlike any other Capcom ports from NES (I think), the Japanese-exclusive GB port of the NES sequal does not redesign the levels to fit the GB screen, making it kind of obnoxious to play. So, I can only recommend this one.
Kid Dracula: I lied when I said I could only vouch for one GB 'Vania.
Link's Awakening: I'm not as in love with this as some people are. My biggest nitpick comes from the limited amount of buttons. Whenever I have to traverse the overworld, I'm constantly entering the menu to swap feather/sword/bracelet, and it's slightly annoying. That's exacerbated by the overworld being so damn dense (which is otherwise a plus).
Mega Man I-V: The first four are largely remixes of the NES games (once again with care taken in the adaptation), but always at some new bosses and stages added in. Part V is wholly original and great.
Metroid II: Seems awkward and a little intimidating at first, but you may find after a while that it's actually pretty intuitively designed as far as where to go and what to do!
Operation C: Not talked about enough. Very good!
Kirby's Dream Land 1-2, Super Mario Land 2, Wario Land 1-2: As good as their reputations suggest!
I can never remember which GB Color games I like are backwards compatible, so these are all OG monochrome games.