Sorry I bet you've all been asked this a million times ! I want to visit Samui when its sunny and hot - when is the best time to do this?

Best time is between December and end of March, although this year there have been many floods during March as you probably know. In general though March is great, very hot, very minimal rain fall, very sunny!

Happy holidays

the weather in July will be quite wet. however, you still enjoy the sun during day time. It will be cloudy with lilttle amount of rain in the morning or evening time.
As per your question, i would rather say the best time to visit koh samui is during December till May.