I always recommend for people to visit is from mid-October through December. I understand this is only a small window of 2 and a half months so if that is not possible January and February are also nice, but much more busy (high season). In May (Though hot), it nearly never rains so it’s a great time for warm weather lovers to visit. Here is itinerary for the 9-day visit
Day 1: Arrival and Phnom Penh Orientation Day 2: Genocidal Museum Day 3&4: Ou Dong former Capital of Cambodia Day 5: Flight to Siem Reap and visit Rolous Group and Small Circuit Day 6: South Gate of Angkor Thom and Angkor Wat Day 7&8: Kulen Waterfall Mountain and Banteay Srei Temple Day9: Floating village and departure Good Luck!!
The best time to visit Cambodia would be from May to October as it's not the ideal touristy time, it would be less crowded and cheaper.
Places to visit in Cambodia-
1. Kampot
2. Kratie
3. Koh Ker
4. Angkor
5. Phnom Penh
6. Koh Rong
7. Poipet
8. Kep Bon voyage!