I have used the following websites for the last 14 yrs as I always check each website for their price.
http://www.asiatrave...el.com/ Though don't use them as much anymore tend to be a bit pricey
http://www.agoda.com/ They tend to be the cheapest but NOT always....
http://www.sawasdee....dee.com/ Along with Agoda can find some good prices. They will beat Agoda on some hotel prices and vice versa. Just saved over $100 booking Anatara riverside thru them over Agoda.
http://www.booking.com/ Good site with some good deals and some hotels don't even need to prepay or put a deposit down and will give you free cancellation
Although not a booking site this is a great site if you are looking for guesthouses or cheap hotels in Thailand....and comes with user reviews
http://www.travelfish.org/