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I also want help with this. Part of me wants to use the Master Ball, just like Kieran, it's the ball in which Terapagos was kept canonically, even if it was only for a while, but on the other hand he broke that ball so...
Dream Ball fits the themes of the game, and makes a pretty big contrast with Terapagos, and it would be kinda poetic to properly keep Terapagos in the ball of dreams when even the supposedly ultimate ball failed.
But Dive Ball fits its color scheme. And Beast Ball doesn't make sense with Terapagos' nature, but the color scheme fits even better.