Best: Marriott Melbourne (Aus) by FAR. Staying as a gold got Chocolates on check in, about 1/2 way into my stay got a nice bottle of red. Lounge was awesome with great food and free drinks.
Worst: Hmm.. I must say I have had no truly bad stays, unlike the airline industry I think the hotel industry in the US has a pretty good baseline.. I would say it would be when I stayed at the Springhill Suites in DT Chicago with the family.. Clearly specified needed a sofabed and they gave us a room where the bed would not open.. Then we got moved to a very low floor with no view.. that said they did compensate us very generously and were very apologetic and nice about it..
Best in the USA.. Hmm... close call between the RI (yes! RI!) in Boulder where I got upgraded to an amazing fireplace room or the Ren in Crystal City Virginia which was just a very nice looking classy hotel with a lounge with an amazing view of DCA...