The Buddha image in Thailand that is most often labelled as the most beautiful is this one called Phra Buddha Chinnarat. It can be found at Wat Phra Si Rattana Mahathat Woramahawihan, or Wat Yai for short, in Phitsanulok Province. I often stop there when travelling up north. It was also one of the first places that I first visited when I came to Thailand. Sukhothai is nearby and it is worth combining the two.