My ranking would look more like this:

1. Chancery Court
2. Grosvenor Square
3. County Hall
4. Marble Arch
5. Regents Park
6. Maida Vale

I think Chancery Court is head and shoulders above the other properties. The building full of gorgeous, old marble, and the hotel has a few entrances you have to pass through that gives a feeling of seclusion and exclusivity. The restaurant and bar are nice but overpriced, but the concierge lounge is fantastic and the staff is generally excellent. In my view this is one of the most luxurious Marriott properties, right up there with the Mayflower in DC.