I've never been to Pattaya beach and don't really care to.
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There are too many nice beaches in Thailand to pick just one. I enjoyed Sairee Beach on Koh Tao for the chillness factor. Railay beach was also very scenic and sheltered.
Bang Tao and Kata beach were lovely beaches for hanging out and walking along in Phuket. I recently went to Koh Chang and enjoyed White Sand beach for the sand and the clean water.