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Emerald's Battle Frontier is still the best post-game battleground. It has the most unique and varied rulesets, subsequent games only made things more and more simplistic. PWT is just regular Pokemon battles coated with nostalgia. It isn't even as good as the Battle Dome tournaments, where the CPU would choose its party based on yours.
Black Tower/White Tree is at least kind of interesting since it has a survival element like the Pyramid (where healing is limited). Probably the most fun that I've had with that game.