We loved the FCC Angkor the most of any hotel on a two-week trip. The beautiful colonial feel, the delicious food, the small but welcoming saltwater pool -- you really can't go wrong here. The Longan room (we stayed here) was lovely, but it seemed so were all the rooms.
The guide they set us up with for our full-day private tour of the temples was incredible. He knew perfect English, and knew so much about the temples -- it really added to the trip and is highly recommended at $90 per day.
The only cons -- the mosquitoes in Siem Reap are intense. Bring repellent!
Best of all, this hotel is only 2 minutes by tuktuk to the main "Pub Street" for dinner. This costs $2 max. Many other hotels are far from town.