I just got back from London last Friday. I generally stay at the St. Ermin's hotel.
From a location perspective, it is really central. There are also really good tube links (St. James station right on the back of the hotel, and Victoria just down the road), and the staff is fantastic. Granted, I stay there 6-8 weeks a year give or take. It is also a Marriott hotel (I have Titanium status, so that helps too).
I have also recently stayed at the Dixon (Bermondsey, also Marriott) and the W in Soho in the last year or so. W is a nice hotel, but the location, right off Picadilly Circus, is a fucking beating. I like the Dixon a lot. It is quieter, and you are a quick walk to Borough Market.
However, if I were in London just for fun, I would stay at the London Edition. Location cannot be beat, and I am a real fan of Editions in general (stays at Madrid and Singapore in the last two years were most excellent).
On this last trip, I had the fortune of being able to go out to Bibi, Mambow, Mamapen (the only Cambodian place in London), and KOL. All were fantastic, but my favorite was Mambow. Holy shit was that good.