I recommend either of the following:
Embassy Suites (Chicago Downtown Lakefront) - 511 N. Columbus Drive
Hilton Suites (Chicago/Magnificent Mile) - 198 E. Delaware Place
While I live in Chicago and have not stayed at either hotel, I did attend functions at both recently, and was impressed. They were equally superb. Great hotel staff attitude, sparkling clean and fantastic banquet/catering. Both locations are central, and close to Michigan Avenue (the magnificent mile).
There are two Embassy suites in downtown Chicago - the one I list above, and another on State Street. I'm not familiar with the one on State Street. It's probably good too, but cannot attest to any experience with that location.