Built in the 12th century, Sikhoraphum features five brick prang (Hindi/Khmer-style stupas), two of which still hold their tops (these were later modified by Lao people who controlled this area after the Khmer), including the 32m-tall central one. Only one lintel remains, but it's a stunner. Featuring a dancing, 10-armed Shiva, it's in excellent condition and is one of the most beautiful pieces of Khmer art ever carved. Below it are the only two Khmer apsara (celestial dancers) carvings still in situ in Thailand.
There's a sound-and-light show here during November's Elephant Round-up.
Located off Rte 226, Sikhoraphum can be reached by minivan (30B, one hour, frequent) or train (7B to 30B, 30 minutes, seven during the daytime) from Surin city. The temple is easy to get to as it's in the town. If you're coming by minivan tell the driver you're going to the prasat so you can get off on the highway instead of in the middle of town.
Sikhoraphum, incidentally, is well known for its gah-lá-maa, the Thai version of caramel. A shop (look for the elephant sign) near the ruins makes and sells it.