In Siem Reap I would stick to the guesthouse's menu. When I ventured for some "authentic" Khmer food, I ended up at the night market with a bowl of some sort of cold, curried, noodle and leg-or-animal soup. If somebody tries to give you something like this, don't eat it. There is also a pizza place called Happy Pizza, which seems to be a chain because there is one in Phnom Penh as well. This isn't your basic Pizza Hut or Little Caeser's, though. No, this place has a special ingredient. They are famous among travelers in Cambodia for adding marijuana to the dough before they make the pizza. This makes people happy. I didn't eat there myself, but I saw it, and they have shirts for sale, which is pretty cool.
My guesthouse had really good food, so I think that is your best bet. They had Asian food but also some foreign fvorites, like spaghetti, cheeseburgers and garlic bread. They also had a very tasty pumpkin soup. Everything was cheap, and you wrote down your tab so you didn't have to pay until the last day.