Best Thai food in Koh Samui.
Best Thai breakfast/late night eaterie "Jay Joo" near the post office in Chaweng. Ask a taxi driver, this place does the best rice soup, salapao and pork with garlic in town.
Best western food
Foodfirst has got it spot on. Bang Por Seafood is a fantastic restaurant serving very inexpensive food right on the beach. Other authentic restaurants worth trying are Pee Chet in Bophut by the traffic lights, Sabieng Lay near grandmother and grandfather rocks and Ranong in chaweng by Pang Bua market.
Koh Samui has probably the best food of any island in Thailand. The fact it is a large island with a large local population means that there are dozens of markets, local restaurants and people selling from motorbike stalls which you just dont get on the other smaller islands.
It does annoy me when people say Koh Samui is not real Thailand. It just simply is not true and is usually said by those who don't actually know much about the country. I have been to alot of the smaller islands in Thailand and yes they are beautiful (more so than Samui in some cases) but they invariably have absolutely terrible Thai food. The workers on these smaller islands are usually Burmese and because of this there is no local restaurants or markets catering to Thai locals.
If you are looking for great food then Koh Samui is probably the best place to holiday in Thailand second only perhaps to Bangkok.