Angkor is hot and sticky throughout the year, but the peak season is November to February, when the weather is dry and temperatures are coolest (25-30°C).
The flip side is that the temples get packed, especially around Christmas/New Year's, and hotel rates are at their highest.
March to May is brutally hot, with temperatures reaching 40°C.
June to October is the rainy season, and outlying temples and the roads leading to them can turn into quagmires of mud. However, this is also when the temples are at their quietest, and it's still often possible to do a good half-day round of sightseeing before the rains start in the afternoon.
Click here for far more travel tips about Angkor Wat.