Bonk's Revenge!! I think this is my 5th time beating it.
As someone who grew up with many AAA Nintendo platformers on the GBA (such as Mario World, DKC, Kirby, Yoshi's Island), I am still astonished at how much soul is packed into this little gem.
It may not be as long as those games, but there's an impressive amount of exploration packed in, even in the more linear levels. The enemy variety is excellent and I still can't get enough of the hamburgers and the butthead Bonk.
Even the bonus levels are fun, and I usually hate mini games.
I think Bonk 1 has the best atmosphere and Bonk 3 has the best exploration/boss fights, but Bonk 2 is overall the best package.