I loved the Raffles/Grand Hotel d'Angkor. Very civilized. Staff has been there for many many years. We had a room in the original building, but asked to switch to a pool-side room (newer building) for the remainder of the stay so it would be easy to saunter out for a swim.

The best part for me was going out in the morning to tour some of the sites & then coming back to the hotel for a nice lunch & swim, which still left plenty of time to go to Angkor Wat for a few hours in the afternoon.
Check for AMEX plat specials here.

One thing to keep in mind about Siem Reap, tons of hotels are going up these days (maybe 40/year? seriously). Personally, I'd choose a hotel that has been operating for a few years (Amansara, Raffles, etc) than a newly opened one. The main reason is that I prefer to have a seasoned staff, not a hotel full of staff just learning how to work in the hospitality industry.