My vote - Mandalay Bay spa
I have been to the following Vegas spas: Mandalay Bay, Venetian (Canyon Ranch), Monte Carlo, and Caesars Palace. Mandalay Bay is my favorite, but Canyon Ranch is also very nice. When I was at Canyon Ranch, it was incredibly crowded in the waiting area. Monte Carlo and Caesars aren't anything special, but they are a little cheaper than Mandalay and Canyon Ranch.