Located 8 kilometers southeast of the city of Chanthaburi, Muang Phaniat is the oldest Khmer site ever discovered in Thailand. It dates back to the 7th century and was inhabited until the 13th century before the waterway changed its course and the city was deserted for 200 years. Then, clay pots and other objects dating back to the 16th century show that new inhabitants have settled here. The place I give the GPS coordinates is the one you will probably visit if you ask a tuk-tuk driver to take you to Muang Phaniat. This is actually Khu Phaniat, an old ound today dry and walls of laterite bricks scattered nearby. These ruins are the most important ruins of the site. You can also go to Wat Thong Thua, a little further away, the temple houses a small museum with objects and artefact found by archaeologists.
This is not a must visit, there are many more beautiful and more complete or restored Khmer ruins in Thailand but as I said, it is the oldest one.