I've lived in Asia for 20 years and been to all of the places you mention between 2 and 50 times. I'd vote for Phuket (full disclosure - I have a condo there so I know it best). It's an easy hour flight which you can often get quite cheap on Air Asia or one of the other discount carriers. The beaches are great there.

I highly recommend Club One Seven. They did a major renovation recently and the place really looks nice. You truly are treated as "family" and they will assist you with any side journeys you'd like to take. I used to stay there frequently before I bought a place.

Siem Reap would be my second choice (been there about 8 times), but it's a little too laid back for me and it can get a little dull in the evenings.