Generally, the best time to visit Angkor is during the cool dry season from December to March. This is when the weather conditions are at their very best. However, it's also when most other people choose to visit Angkor, so you'll find hotel rates at their highest and the temples at their most crowded. Our suggestion would be to come in November. There's still a slight chance of rain, but storms usually only come in the late afternoon, which is a good time to rest up from a morning visiting the temples anyway.
Early in the dry season is probably the time to see the temples at their very best. The great temple of Angkor, as well as many others in the complex, is surrounded by large reservoirs that fill with rain water during the monsoon and are often completely dry by March or April. To see the temples reflected in these pools filled with lotus flowers, you need to visit in November or December.