While I would agree with snags on Pattaya (unless you want the local prostitutes pestering you the moment you step outside your hotel), I happen to like Patong Beach (Phuket) � even if it does have its seedier side.
I would recommend Le Meridien hotel, located about 3 miles south of Patong Beach, it effectively has a private beach and as a result you don�t get pestered by the locals.
As for the time of year to go, the rainy season lasts from April to October (our summer), when prices are lowest. A number of hotels in Thailand offer two weeks for the price of one during the rainy season, and if you go at the end of such offers � you will normally not see too much rain and save a bit of money.