Visiting London is best in the summer. Remember airco is less common and less powerful than in the USA. But London in cold rain is NOT nice. Although the museums are very good of course, they're not exacly next to each other. And a lot of the "unforgettable" views are outside, like parks, the London wheel, buildings, palace grounds, the Tower. The subways are going to be hot and packed with tourists... but that is better than wet, humid and packed with commuters.
For a spectacular building: the Marriott County hall is great, check the room view though, some views are on the Thames, Big Ben en the Wheel (NICE!) , some are not (dreary..). It is not the budget option though! Also not something with kids, prams etc. Most guests are business people. Marriot Kensington is in walking distance from Hyde Park, and shopping areas, and although a little out of the way, this would be a good budget option.
If you are from outside the UK, do not count on driving in London centre (they have a toll system called congestion zone, and you can't park for love or money) so your hotel needs to somewhere in the centre. There are great value for money places several tens of miles out of the center, but these are not useful for a tourist visit to London. If you are a business person - all hotels in the center are ok, pick the one closest to where you need to be or the one closest to your airport connection (Paddingron, Victoria, Piccadilly Line...)
Tourist Beware! Give yourself much extra time, this Summer. The Olympics are only going to make a traffic and crowds worse. Am planning on staying at County Hall in May and then getting back before the Summer crowds. Hope all the construction will be done on the highways, but really doubt it!
As "locals" ie live in England but visit London for business and personal outings -- our preference is JW Marriott-Grosvenor House in Mayfair. Slightly more expensive but classy and special. Back up option Marriott Grosvenor Square oppositie the AMerican Embassy. Especially good if you wish to explore the delights of Bond Street shopping. Another option if you have a family with you is 47 Park Street where you can get a 1 or 2 bedroom apartment -- can be self catering if you wish. Both close to Hyde Park and central to all main areas. County Hall is equally goo close to the Houses of Parliament but you will need to reply on black cabs a little more often. Timing - agree with the comments about weather. Best times mid May - August. Make sure you take an open top bus tour - really good way to see all the major sights. Can include a section on the Thames by boat.
Finally can report that currently the roads in London are a mess. All dug up attempting to sort things ahead of the Olympics 2012. Not sure they will make it ! Expect chaos.
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