Jerry is quite right everyone will have a favorite but Sally and I liked the West India Quay Marriott at Canary Wharf. It is a bit away from the heart of London but the tube will get you anywhere. The hotel is quite, there are great places to run, nice restaurants and appeared to be very safe....Bill p.s. Enjoy there is no city like London!
I concur with Theolejudge that the West India Quay is worth consideration. We spent a week there and really enjoyed it. Its about a 10 minute tub ride to downtown. I use Marriott Rewards points (Cat 6) for our stay. The light rail train also runs right next to the hotel and is a nice option for parts of London.
I like the Marriott County Hall the best. However, I also liked the Kensington (but not as much), the Marble Arch, I got an "upgraded" room which was fine, but I'd seriously like to know what the room would have been like had I not been upgraded. Get an Oyster card at a tube station, as long as you're near a tube station or on a bus route, you should be fine. Have fun.