SihanoukVille, Cambodia is one of the best beaches in Cambodia. A bit raw, with the poverty and such, but probably the best you are going to get. Everything is dirt cheap, a quality room will run you at most $20, food a couple dollars for a feast, etc. Best during Dec/Jan/Feb, plenty of sun, lower humidity, not much rain. Problem is, there isn't much to do besides hang out at the beach and drink (plenty of bars around with CHEAP brews).
Personally, Thailand would be the better choice. Things are a bit more expensive there (depends on where you go in the country, though). You have world-class beaches, great food, moderately priced accommodation, hot women, great golf, you really couldn't ask for anything more! Lots of things to do wherever you go, be it Phuket, Koh Samui, Pattaya, etc. Phuket is the most expensive by far.
If budget has no limit, and you just want some AWESOME beaches, resorts, and watersports, then the Maldives are the place to go. You can get a nice water bungalow (you are in very shallow, beautiful water surrounded by all kinds of cool wildlife) for $300-600/night, at a quality resort. The upper-class resorts there will run you well over $1000/night..before any food or entertainment/activies is factored in.
