I spend 3 winters touring SE Asia so I've been to Bali, Kho Samui, Phuket, Siem Reap, etc but it was all in the 90's so a bit dated. I'm also more of a $5.00 room person, as I was gone for months at a time.
Really enjoyed wherever I went though. Bali is sort of the Australians version of Hawaii, so very touristed but still a beautiful place. Very Hindu, which is interesting. Great crafts/shopping.
S. Thailand also very touristed, don't know if things are quieter now with the Muslim separatist issues down there. Great clear warm water, nice beaches in both places and both Thailand and Bali are very well organized. Bangkok is a very big city, much like London is for England. I liked getting away from the crowds in the ferries, and if they havn't filled in the canals yet, blasting through the narrow canals in a long tail boat powered by a car engine on a gimbal is cool.
Vietnam might be another place to consider, I was there before tourism really started, but no doubt everything is organized now with the beach resorts, etc. Culturally it is also quite interesting with the Chinese/French/American history there and there is lots to see on day trips as the country is basically a long narrow beach/paddy/mountain.
Malaysia also has it's share of interesting things, never spent beach time there though.
What else? Food in Thailand utterly rocks for me, I could spend a winter there just eating. Vietnam is very good, Malaysia ok, Indonesia pretty workmanlike for the most part.
Might be a bit hot in that area still in Sept.