Are we talking original Game Boy, or also Color? I'll mention less talked games.
Mole Mania is still ridiculously underrated, and an absolute gem of a GB game. Looks great, plays great, sounds great.
Final Fantasy Adventure is another I love. Great music, I love the different ways you can build your character (magic all the way!) and I can't get enough of it. I like it far more than the reimagination that is Sword of Mana on GBA.
Talking about RPGs, FF Legend III too. While I like the previous two, I enjoy this one more for being a bit more straightforward in its mechanics... And being able to jump.
Operation C is a fairly impressive attempt to make a Contra on a GB. I like the weapons more than on NES, and it has a color hack that works great.
Cat Trap and Solomon Club are two puzzle games I enjoy quite a bit. And both Mario Picross are delightful.
Ninja Turtles III - Radical Rescue is a pretty interesting game, as it has you exploring a large maze trying to rescue your brothers. It could be named a metroidvania of sorts.
While not favourite, the title simply named "Looney Tunes" it's a pretty respectful licensed game. Each stage stars a different character with different gameplay style. Far better than I would expect seeing other Looney games out there.
And I agree, Motocross Maniacs is fun.