Shining Soul II for the GBA. It's a real-time RPG set in SEGA's Shining Force series, and it got terrible reviews. Nevertheless, it's one of my favorite games, both for the GBA and just overall. The storyline is weak as hell, some animations are pitifully limited (particularly those of the first boss, who has a two-frame, 360-degree axe spin attack) and the enemy AI is retarded - it takes a straight line to you, even if that means trying (and failing) to walk through walls - but the game still manages to be both challenging and awesome. Multiplayer - both co-op and versus - is great, too. I think it might actually be the best game the GBA has to offer in terms of multiplayer. And it never stops being fun to try to continue levelling up your character. Shining Soul II also has what I think is one of the better weapon and spell systems I think I've ever seen in a real-time RPG.