I've gotta go for Agatha Christie. To my knowledge, King Edward VIII & Kim Philby may have stayed and 'socialized' at the hotel, but Agatha actually was supposed to have done some writing there. The hotel plays up her visits there the most...I understand that there is quite a bit of Christie paraphernalia scattered around the common areas & a room that the hotel claims was her favourite.
I agree for the same reasons (Agatha Christie wrote The Murder on the Orient Express at the hotel), though Kim Philby used the hotel for clandestine meetings and King Edward VIII merely stayed there