I can't say that any of my 2012 Marriott stays could fall into the "worst" category, however most of my travel is domestic, so I never have any unrealistic expectations.
For the "best" I would choose the Amsterdam Marriott, where we had a six night award stay in late summer. We followed this up with a six night award stay at the Rive Gauche in Paris. From my perspective, this property is very underrated. While it is true that it is off the beaten track, this is offset by easy access to reliable public transportation as well as a lower cost, be it in points or cash. However, the location of this property is why I am placing it slightly below the AMS Marriott.