Nobody has mentioned Grosvenor House, A JW Marriott Hotel. It's a luxury hotel facing Hyde Park. Although the name is similar to the London Marriott Hotel Grosvenor Square, they're two different hotels, several blocks apart.
Grosvenor House, A JW Marriott Hotel is Category 9, so it takes the same number of points as London Marriott County Hall and London Marriott Hotel Grosvenor Square. But, if you're not paying with Marriott rewards points, the JW is usually much more expensive than the other two. So it's a good use of points for those who like to maximize the value of points.
Another luxury hotel, already mentioned in this thread, is the London Marriott Park Lane. Although it's branded as a regular Marriott, not as a JW, I'm told it's similar to a Ritz-Carlton. Park Lane is also Category 9. It's usually about as expensive as the JW if you're not paying with points. But it's a smaller boutique hotel. One big advantage is that there's an underground station at the same corner as the hotel.
London Marriott County Hall is a good tourist location, especially if your plans included the London Eye, Shrek's Adventure, Sea Life Aquarium, and the London Dungeon (all of which are at County Hall). Guests who get rooms with great views of the Thames or Big Ben love those views. But a Category 9 booking does not guarantee such a view, unless you pay a significant up-charge.
There are other Category 9 properties — St. Pancras Renaissance Hotel, London Marriott Hotel Marble Arch, and Threadneedles Autograph Collection. I haven't been to any of these, but I would not consider them for a London vacation for various reasons.
A reward booking at St. Pancras normally gets a room with only a queen, unless you pay an up-charge. If you're Platinum, you have access to a great lounge. If you're Gold, you only get breakfast in the restaurant. There's excellent access to trains, underground lines, and other transit.
I understand that Marble Arch is a good, fairly typical Marriott hotel, but it takes as many points as better hotels.
Threadneedles is away from the most of London's attractions. It's probably a good hotel for business travelers who want to be where it's located.
For me, the most appealing Category 9 hotel in Central London is Park Lane. I haven't stayed there yet, but I plan to on our next trip to London.