Dragon Ball Z: Legacy of Goku I for the GBA. Now this game got heavily criticized for lackluster gameplay including fetch quests, enemies are way too difficult requiring spamming tatics, and you can glitch when flying and get stuck, and that it takes a lot of liberties with the story. However, it was one of the first gba titles I managed to beat, and I thought some of the music was pretty good, ok graphics, and spamming bosses and finding new tricks motivated me to replay this game a few times after I originally beat it. Unforunately, you will struggle with combat until you get Solar Flare, which is easily used to stun enemies for long periods of time, and used for 3/4ths of the game as the primary way to win.
While I think Legacy of Goku II and Buu's Fury are far superior, and that I don't really find myself going back to Legacy I all that often, I still applaud the game for what it did, what it tried, and what it did right. It was not only one of the first GBA titles, but the first in the Legacy of Goku series, so being lackluster is excuseable. Plus, I am sucker for voice-acting and FMV cutscenes in my video games, I loved how you heard Goku yell "KAME-HAME-HAAAAA!!!" upon firing a fully charged Kamehameha, how he yelled "aaaarrhhhh" upon dying, and the fact that the game had FMV's taken from the anime^_^ It's a guilty pleasure, but not a bad game - just a game with some flaws that its sequels successfully ironed out.
--
That said, if I should pick a game, I'd say Chain of Memories. I love how that game forces you to think strategically instead of just button mashing till you win, the sprites are beautiful, the FMV cutscenes are impressive, I love the story, I absolutely ADORE the remake..... and everyone else hates the game for introducing the card gimmick^^'