It depends on budget and what location you prefer to be: in the middle of tourists spots and close to the public transport or not.
Favourites:
The Waldorf Hilton, for stays close to the Theatre west end district
Curio The Trafalgar St. James, right on Trafalgar Square close to the public transport
Conrad London St. James, Good location and steps from an underground station
DoubleTree London - Islington, the best budget alternative I found in central London, good also for public transport