Wat Phra Kaew, Chiang Rai. According to legend, the Emerald Buddha statue was (re)discovered when a bolt of lightning hit a chedi (stupa) on the grounds, cracking it open and revealing the Buddha inside.
The temple grounds also include a compact but excellent two-story museum.
A building a the rear of the Wat houses a near-exact replica (1mm shorter!) of the Emerald Buddha. The original is now housed in Bangkok in the temple of the same name, within the grounds of the Royal Palace.