I live in Chicago, and being an HHonors junkie, have chosen to stay at one of their downtown properties for various special occasions.
The Drake is considered to be the most upscale, and you can't beat the location. It is on the Magnificent Mile, and close to great restaurants, sites, shopping, and nightlife.
The Palmer House is an older, classic hotel that has a truly charming, ornate lobby area. Rooms have average amenities, and bathrooms are small.
The Hilton & Towers is also very nice. It is more modern, but the rooms are still just OK. However, the nightlife in the area leaves something to be desired. But of course, you can "cab it" anywhere.
In short, if I could choose any of the three w/o regard to cost, I would take the opportunity to visit the Drake, which is a famous Chicago landmark. Have fun!
--BBQ "Brisket ain't so great" Pork Sandwich