Ensure as a minimum that you visit Siem Reap, and I would definitely take a private or small tour guide to Angkor Wat to marvel at the temples and ancient ruins, team it with a few nights in Phnom Penh finishing with a beach stay in Kep or Sihanoukville.
Whilst staying in Siem Reap we had a private local guide who took us exploring the ruins and temples of Angkor Wat where we were able to stop and take in the sights, he has to be the most knowledgeable guide I have come across but, witnessing the sunrise at Ankor Wat was truly a magnificent experience and well worth getting up at 4am to be there.
The best time to visit Cambodia is between November and May where the rainfall is at its lowest and still enjoys warm temperatures. However, the climate is generally hot and you will incur some humidity all throughout the year.
