Guys, I know there are a ton of Marriott London posts here, but every time I see one I feel like I need to drop in and say that the RCC is possibly the best Marriott I've ever visited in the entire system.
When I saw Marriott was adding an 8th category, I instantly thought of RCC. Sure enough, it's an 8. If any hotel deserves an 8, this one does.
Over the course of the past year, I have hit nearly all of the main London Marriotts: I even went out and did Maida Vale on a short business trip once, just to say I'd done it.

Of the central, top-price Marriotts, I'd go back to the RCC every time. Maida Vale and Regents Park are more of a cost/benefit discussion: it doesn't really make sense to compare them against the Category 8's.
The JW was also good, but not as good as the RCC.