Castlevania, it's not even a choice. Golden Sun is far too text-heavy (meaning far too many dialogue scenes and simply not enough 'I'm in control and can explore' scenes), and Minish Cap is a total insult to the entire Zelda series.
This is a toughie.
Castlevania has in many ways, i feel, defined the GBA. The games are all brilliant extensions of SotN.
Zelda on the other hand, is a whimsical, beatifully rendered, creative masterpiece. Twilight Princesses' reuse of most of the dungeon items could take a page from the unique items in Minish Cap.
Golden Sun, what can I say? I don't get it. I love RPG's, but I played it for about 4 hours and almost died of boredom. I put it on VBA to frameskip through the slowness, but it didn;t help much.
Overall, I'd have to go with Castlevania.
Minish Cap is a total insult to the entire Zelda series.