While the Drake is nice enough and certainly a Chicago icon, for the type of money you are talking about, I would recommend
1. The Peninsula
2. Four Seasons

Both have lovely service and some nice deals on free amenities and the like. My personal favorite is the Peninsula and I think you would find similar concensus on the luxury hotel forum on flyertalk.

If you are coming in the summer, the Peninsula has a lovely terrace restaurant and bar outside as it does a fabulous rooftop pool and spa. Generally, the Peninsula is where hollywood types stay.