Nanu.
He may only be the 3rd Kahuna, but he might be the most powerful when he puts his heart into it; after all, he is a Faller, a Kahuna, he is not a coward since he hangs around team Skull, and he might have connection somehow to Arceus, the reason I say that is because immediately after you defeat him, you can start to collect the Elemental Plates. Plus he is also my favorite supporting character from all the generations.