Onsite I'd say you'd probably best stick with Saratoga Springs or Grand Floridian. The Marriott World Center and Gaylord Palms have nice offsite spas, while we havent' yet had a chance to check out the spa at the new Ritz-Carlton/JW Marriott.
This is all according to my wife, of course, who is a confessed spa-junkie. She really loved the spa treatment and facility at Disney Institute (now Saratoga Springs); that would be her recommendation, with her second choice being offsite at the Canyon Ranch spa in the Gaylord Palms resort complex. She's not a big fan of the staff or prices at Grand Floridian.