Giratina>Dialga>Palkia easily, though all three kind of suck. I don't get Nintendo's passion with making dragon types that don't look like dragons. Maybe I don't know mythology well enough, but the DP and BW trios just don't look like dragons to me. Regardless I also kind of thought that Palkia and Dialga's types or lore should be switched. It makes more sense to me that water and time both flow, I guess space is expanding, but water doesn't expand. I'm probably just being picky here. Dialga's design is a little better than Palkia's overall. I like it's coloration better and it's neck isn't as stupid looking. He also didn't piss me off in Brawl. Avoiding his beams and dealing with less ground isn't nearly as bad as the controls flipping.
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