The temples of Angkor are Cambodia's star attraction and many travellers prefer to visit when the weather is at its best. And that would be the months of November to March, when temperatures are cool and days are mostly dry - great for exploring ancient temple ruins.
However, this coincides with Cambodia's peak season, which means bigger crowds, less hotel availability and higher prices. If you would prefer to visit when crowds are thinner, consider going in the shoulder monsoon months of June or September/October. It's likely to be wet, hot and humid but the temples will be a little quieter than at other times of the year. Plus you'll get to see Angkor Wat when its moats are filled with water.