So I wrote a short essay on this once but to be brief, it really depends on what aspect of play you're trying to emphasize. I think some of their best stages are FoD and Smashville, and I've heard talk that Green Hill Zone is their best stage from multiple knowledgeable people (the only reason I don't personally endorse it is just because I don't know enough but in theory it sounds great for them). By contrast I'm of the opinion that Delfino's Secret is flat-out their worst stage on pretty much every stagelist that includes it, and I avoid it on principle except in very excepting exceptions.
Also, if I'm not mistaken, Fountain has one of the largest vertical blast zones on most legal stagelists, but this doesn't matter too often (except against heavy fast-fallers) because platforms and also Icies' up smash and up-b are strong as heck.
