Without doubt The Dorchester. No question. No competition.
Second choice The Connaught.
Third choice The Savoy.
It possible try and book a 'Dorchester Weekend Indulgence' package through LHW. You should be able to get it for betweem 300 and 350 pounds per night.
If you want something less prestigious and more intimate I would recommend Blakes. It is a little out of fashion at the moment, but still maintains that air of conspiratorial indulgence, that only a small hotel can manage.
Nick