I would have to add that The Berkeley also now boasts the only retractable rooftop hotel pool in London; it is simply incredible, perhaps even better than it sounds! Also, the Blue Bar is definitely one to check out!
I concur with David's views on the Dorchester, a fine, fine property. It was there that I shared the hotel's (very) modest gym with the crown prince of Brunei (fitness fanatic my god). Yet, whilst you hear utterances of the fame of those staying there and the occasional group of extremely large gentlemen with earpieces in the lobby, the service remains extremely personal and is in not way negatively affected by the demands of other clientele. Also, the bar at the front is great, the one at the back could do with some refurbishment of both the bar and the clientele (mainly the latter...seems to be a problem across the board as the MO could use with one as well).
Finally, I'd have to add one to the small boutique hotel category, The Duke's. Located right off St James this hotel is fabulous. The concierge and reception staff are simply the best at aiding you with all your queries and requests. The Duke's is nicely tucked away down one of London's narrowest streets (don't worry not a problem as it just opens up into a courtyard). The layout of the hotel is slightly strange as it is a mishmash of extremely old townhouses. The Duke's also can lay claim to one of the best martinis in the world. Their martini (and cocktail) bar is truly world class and was reportedly where Ian Fleming came up with the idea for the original Vesper (from the book, not the movie). All in all it is truly a classy establishment that allows for the best of stays be it for business or for pleasure. For those who appreciate a bit of quirky info, Princess Di used to also *ahem* frequent here, weekly afternoons off, I do believe.