Well, this is quite a tough one really isn't it? Palpitoad has pretty much already proven itself to be a very powerful member of Ash's team, though with Donphan, he's been a member of Ash's Pokémon for quite a while now, and it certainly has proven what it can do.
As for now, I'll probably go for Palpitoad anyway, seeing as I somewhat like it's personality. I always just thought, Donphan was just... there. It only had it's chance to shine in the Battle Frontier, in which we just saw it evolve it had a few battles but that was it. It didn't get much time in Johto either.