Soundvoyager is part of the bit Generations series of abstract, experimental games. This one in particular is all about sound, and it's one of the few games I know of that you can play blindfolded (with earphones).
Gradius Galaxies is a solid entry in the series. Similar to Gradius Gaiden, you can choose between four ships with different powerups, so it has good replayability. Shoot stuff, power up, and destroy the core.
Rhythm Tengoku/Rhythm Heaven Silver is basically Wario Ware with rhythm. It has a similar brand of whimsy and wackiness, with games like plucking chin hairs from onion heads, baseball in space, and marching in bunny costumes. Besides being an enjoyable, replayable game, this is also my go-to game for testing input lag/latency on emulation devices.
GT Advance 3 is the latest and perhaps most polished game in the series. It's Game Boy's "we have Gran Turismo at home," but in a good way. You race to win championships, unlock cars, mod them with parts, etc.
Konami Krazy Racers is a competent Mario Kart clone. The game's original Japanese title is Konami Wai Wai Racing, so like the Wai Wai World games on Famicom, it once again features characters and locations from various Konami franchises: Metal Gear, Castlevania, Twinbee, Pop'n Music, Goemon, etc.
The Nightmare before Christmas: The Pumpkin King is a surprisingly decent metroidvania for a licensed game.
For homebrew,
Apotris deserves a mention. It's a polished game of Tetris with various modes.