My vote goes to Wario Deluxe as well, it takes longer to get going in terms of speed, compared to the other ones, but once it does it's every bit as frantic as the others.
Plus of course there's the major factor of it really feeling like a final battle, especially when Wario starts to take the prompts away from you for a few turns, that's something no other, regular stage in the series has ever done and it's just all around a very satisfying and even climactic final stage.