Had Khom (Coral Beach), north coast of Ko Phangan (click to expand)I put together a few shots from the site of what I regard as some of the more pristine beaches in Thailand. Forget about dictionaries - let's define that as having nice sand, clear water, cleaned of wet-season rubbish and fishing junk, with no development or with unobtrusive development and never being crowded (at least away from public holidays or weekends for those beaches easily accessed by Thais).
This is just a sample there are others which fit the description - plus quite a few really nice beaches which don't meet the no/low development rule.
Donald Duck Bay, Similan Island 
National Park Headquarter Beach, Similan Island 
Nui Bay - southern Ko Lanta
Ao Si - Ko Jum
Relax Bay (Hat Phak Naam) - the Phi Phi knockers never make it to the lovely and uncrowded east coast bays. Image Louise F.
Leela Beach, back of Had Rin, Phangan
Bamboo Island near Phi Phi - this one is a bit contentious because it can get a bit busy with daytrippers in dry season. Looks pretty sweet here - image Louise F.
Oean beach near Poseidon Bungalows, Khao Lak (image Similantour)
The only beach on Ko Lialiang - Image - http://www.laoliangresort.com/
Had Thong Reng, east coast of Phangan - site of the new Tree House moved from Ko Chang
Ao Pudsa/Tubtim - Ko Samet
Bang Bao, Ko Kut
Loh Paret, Ko Yao Yai
Ao Jak, Ko Tarutao
Ao Malaka, NP headquarters beach, Ko Taruato
Eastern beach at NP headquarters, Ko Adang.
