Azure Dreams for the GBC. It's a port of the Playstation game with a lot of elements removed and some changed, and some would say that it's a Pokemon ripoff (though the equivalent of a Pokeball is one of the rarest items in the game), but I've played the !@#$ out of that game and loved it. I got somewhere around 80 hours in before my official cartridge died on me, so I only managed to finish the freshly-restarted game via Lameboy and its speed-up function.
Without the speedup button, it's a simplistic RPG that goes way too slowly to be enjoyable. With a speed-up button, it's enjoyable enough to at least finish once through. I'm saddened that all of the Youtube videos I've seen are from people who have no idea how to play the game. I'm not being egotistical there; the game basically says very early on "Here is how to change my (your first monster's) AI level from 'I assist you by adding slightly to your attacks' to 'I directly attack enemies for you'" and none of the videos I saw bothered to heed that advice.
I think my love for the game is partially because I never played the PSX version, so I never got to see the "full" version where the main character woos girls and such. I don't care, I'll merge a ShoTroll with a Souvenir to create a Berserk that'll rip your effing head off in one turn. [/geek]
EDIT: This was also one of the few "dual-mode" cartridges, with a black plastic case, which meant that they would work on an original Game Boy (and Super Game Boy!) but had color enhancements for the GBC if played on one.