Hilton HHonors Amex is good, but my favorite is the Starwood American Express. It covers Westin, Sheraton, Le Meridien, Four Points, and more. No blackout dates, capacity restrictions, etc.
Best additional feature is the ability to transfer your points directly into most airline miles programs at better than 1:1.
Learn more about it here:
Why Hotel Credit Cards are the Best Kept Secret for Travel Rewards