From the April 2006 issue of Conde Nast Magazine, two Chicago Hotels earned the #1 and #4 spot in the Urban Hotel Spas category:

#1 was the Peninsula Spa http://chicago.peninsula.com/pch/fac.html
A description of the Peninsula Spa from City Search:
The Scene
Located on the 20th floor of the Peninsula Hotel, the elegant spa is a welcome retreat from the hurried scene on Michigan Avenue. Divided between two floors, the spa is open and airy, decorated with soft, natural colors and striking flower arrangements. Immaculate changing rooms are downstairs at pool level. Upstairs, the uncluttered waiting lounge overlooks the pool with impressive city views through towering windows.
The Services
The level of service and expertise are what you'd expect from a hotel of this caliber. Opt for a 60- or 90-minute Swedish massage--perfect for overall relaxation--or go for one of the signature treatments: The Thai massage is a 90-minute, clothes-on procedure that incorporates gentle rocking, acupressure and assisted stretching.
The Details
Whirlpool (co-ed, fee), steam rooms, pool (co-ed, fee), exercise equipment (co-ed, fee), Pilates (co-ed, fee), fitness classes (co-ed, fee), yoga (co-ed, fee). Massage, body treatments, wet treatments, facials, hair removal, manicure/pedicure.

#4 was the Four Seasons Spa http://www.fourseasons.com/chicagofs/spa.html

A description of the Four Seasons Spa from City Search:
The Scene
The eighth floor of the Four Seasons Hotel boasts 8,000 square feet of pampering heaven. Treatment rooms are soundproofed for privacy, and the gracious staff makes you feel welcome at every turn. Wrap yourself in a thick, silky robe and you'll never want to take it off. Prepare for your treatment by relaxing in the lavish spa lounge: The stresses of the outside world melt away as you begin to unwind with fresh fruit, minted water or herb tea.
The Services
Signature treatments include the invigorating crushed pearl and lavender body polish and the Four Seasons aroma blend massage. The nail salon features two whirlpool pedicure chairs with built in heated massage and two manicure stations. Expert massages are available in several styles including Swedish, sports, stone and couples massage.
The Details
Massage, body treatments, wet treatments, facials, sunless tanning, manicure/pedicure.

-
Mrs Sweet Willie and I have visited the Peninsula Spa. 90 minute massages followed by 30 minute reflexology. YUM !!! ^