5 star all-around
I go to work often in London, and the best hotel experience I had was the Sofitel St James. Now, I was extremely lucky that my company was paying for it, because it is 5 star and its price reflects that, of course.
The location is right next to some nice opera places, and the St. James Park and Buckingham Palace. You are walking distance from the Piccadilly Circus tube station. I had a standard hotel room, and the bathroom was better than I could ever hope for in my own house. The bathroom has white french doors in the hallway, a bathtub and separate standing showering area enclosed with glass. If you're jetlagged, the gym is open 24 hours, and there are trainers there during limited hours (I think 6am to 6pm). If you like to go jogging, you can go to the St. James Park, just a block away and down some stairs. A nice circuit to run is going south in the park down to Buckingham Palace and back to your hotel. Great to be able to get fit and sightsee at the same time!