A Superb Hotel
I second what Tina-Perth and others have written about the Raffles LeRoyal Hotel. Landmark rooms are small but nicely furnished and very comfortable.
The gourmet French restaurant serves, well, a gourmet repast. Expensive by IndoChinese standards but well worth the cost. The less formal eatery -- with both indoor and outside seating -- serves an elaborate breakfast and decent meals all other times of day. (I killed a mosquito on my outdoors table one evening, the only such beastie I encountered in Cambodia.)
LeRoyal figures in many books about the 1975 Khmer Rouge take-over. Unlike other "historical hotels" -- the Rex in Saigon (HCMC), for example -- LeRoyal is worth a stay because of its location, its fine architecture, and its amenities.
Unique Quality: Nice grounds, nice pool, good restaurants, many amenities.
One caveat: A check-out clerk asked me if I wanted the hotel's car to take me to the airport. I asked how much. He said -- I thought -- fifteen dollars, added to my hotel bill. Since a junky taxi on the way in had cost $10, that seemed reasonable. I didn't check my bill till I was flying to Siem Reap. The ride wasn't "fifteen," but 50 -- US$50. I should have suspected something. The "car" was an eight-passenger BMW limousine, the most luxurious car I've ever been in. (The ultimate joke may be on the hotel, however. BMW, like Mercedes and most European cars, has a terrible repair record.)