I just visited Siem Reap.
Cuisine Wat Damnak is a safe bet for a more advanced take on the Cambodian kitchen. I enjoyed the whole menu but missed the taste sensations. The most memorable dish was actually a small dessert amuse with three types of bitter and acidic fruits.
Mie Cafe is a newly opened small restaurant by a young, local chef trained in Switzerland. He buys all the groceries himself every morning in the market. He mixes European with truly local in a way that works. Tuna tartar, the salads and Fish Amok were all great. Prices are low and service is fantastic by Cambodian standards.
I tried many other places too, both local eateries and upscale hotel restaurants, but nothing was really memorable except for the drinks at Miss Wong.