For those with lots of time and patience.

User Rating: 9 | Dragon Quest III GBC
There is nothing new in the game, just your everyday old-school RPG, although it's a very memorable one. Dragon Warrior III is a rather hard game, requiring lots of level griding in order to survive certain boss battle, or just random encounters in general. But hey, it's pretty much the case with all Dragon Quest/Warrior games, at least the ones I've played. If you've got lots of time to spend, it will grow on you.
Not the best of graphics, but then again it's for GBC, so what do you expect? And I've definitely seen much worse compared with this.
For gameplay, there are many choices regarding party members, and it's quite fun to try out the different combinations. You can always go solo, and that gives it the extra bit of challenge (already plenty in the game) that you desire.
This game is also quite long, with most of the time spent leveling up, but most of the time I didn't mind it at all.
To sum everything up, this is definitely the best RPG for GBC.