The Sheraton offers OK views... it's in a wierd location though, off of Devon Ave and Mannheim road. The hotel isn't that big, but you'll definitely see the tails of the aircraft.
The Clarion on Cumberland is OK if you like to see them approach...
The Hyatt O'Hare doesn't allow any views, except for the approach...
The Westin would offer you a great view of the North-Field and south field... pretty unobstructed.
and just across the street is the Marriott Suites O'Hare, and if you get a high floor, you can get a decent view of the south field.
Hyatt Rosemont is on the corner of North River Road and Higgins I think... no where near any good spotting, unless traffic is coming in on 22L
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