I've been at the Regents Park a few times and it is great for those with a car, as parking is free!!!! They have a useful concierge level as well. However, this hotel is a little more removed from the action than any of the others.
Grosvenor Square is in a good location and is a prime property, however the rooms are a little small. One can walk to many points of interest from this hotel. Their afternoon tea is one of the best in the city!
We've also been at Marble Arch, and have not found it as comfortable. It is a little out of the beaten path as well, but not as far removed as Regents Park.