Just returned from a trip to Cambodia, and I've seen few postings on chowhound for this area, so here's some tips:
In Siem Reap (Angkor Wat), the shacks near the temples all served decent food. Nothing too exciting but better than most tourist traps. In Phnom Penh, we just had street food at random. A lot of travelers go to one of the "special herb pizza" places, 4:20 if you get my drift. Unfortunately, we didn't have time to.
Sihanoukville, however, was a highlight. It's the main beach village on the southern coast. The seafood is some of the best we've had anywhere. On the main beach, we sat all day as women came by with trays carried on their heads: fresh tiger prawns, sand lobsters, squid, various spring rolls, all bbq'd to order. You can bargain on the price, but you're talking 25 cents for some of the best lobster you'll find. We were told there was no solid export industry for seafood yet, so you get the best stuff for cheap. We also ate at a restaurant here, Koh Pas, I believe, a big special kinda place. The place was empty at lunch, but we had a wonderful meal. Cafe Claude was a friendly fancy place but the food was just "good French food for Asia", and overpriced.
Back in Thailand, we stayed on Ko Chang Island. The food here was hit and miss. One restaurant, Chow Lay Bang Bao, on the alley in Bang Bao fishing village on the southern tip, was worth seeking out. Get a table over the water an hour before sunset. This is the seafood restaurant/tropical sunset view experience of your dreams. You choose the live seafood, but note that most places here price by weight -- a sticker shock after Cambodia.
Bangkok. We tried one of the restaurants(Issan northern Thai food place) recommended by the Wall Street Journal, but were disappointed. Forgoe restaurants in Bangkok, and just dive into the street food. It's all good, cheap, and pretty clean compared to other places we've been. You can find all kinds of unusual things, including the bug carts: scorpions, grasshoppers, maggots, etc.