I've tried the spa at the Intercontinental and Westin. But their neighbor, the Ritz Carlton has them both beat. The Ritz has perhaps the finest wet facilities of any spa in the city. Everything is oversized so you will never feel cramped here. Great service too and probably the highest prices.
I especially love the jacuzzi beds. I've never seen this anywhere else. You have the normal jacuzzi basin, then you have these molded recliners with jets built into them. So you lay down and relax and the jets pummel your body, mostly on the back and shoulders. The only drawback is that like every other hotel outside of Japan, the jacuzzi isn't hot enough. The steam and sauna however are hot enough and well sized.
They gym is nice and well equipped and the pool is good too. To me this ranks very well with the Grand Hyatt for the best hotel fitness spa facility. The GH has a better pool of course and has yoga classes, but the Ritz is better in most other ways.