Please use this website if you can and you support me and this website.
I use R24 and they are very competive, not always cheapest but in most cases very good rates. Also beware of websites with lower rates but once booking, they add taxes and other charges to it. (Not the advertised rate)
The best rate depends on the hotel in question, for one specific hotel it may change a lot between websites, one websites may be cheapest for one hotel but not other hotels and vice versa.
www.phuket-info.com/hotels.htm