Cambodia was at its height around 1200 AD. At that time it included some of present day Vietnam and Myanmar. It has both Hindu and Buddhist influences and the temples of Angkor Wat reflect this.
But its recent history is filled with war and turmoil. Following the Vietnam War, civil war killed millions of people. The country is still recovering.
If you can only make one stop in this beautiful country, then definitely make it Siem Reap to see Angkor Wat. However, I also challenge you to stop in Phnom Penh to learn about the war and the killing fields.
Spend at least one day in Phnom Penh to see the killing fields, S21 prison, and the palace. S21 prison was hard to follow on your own so I recommend either a tour or paying extra for the audio guide. Please note, the palace requires shoulders and knees to be covered; but unlike other stops in Indochina, a scarf does not suffice.
I highly recommend Okay Boutique Hotel. It is centrally located, has amazing wood work, a roof top pool and restaurant and very comfortable rooms.
First They Killed My Father is a wonderful book (and movie) of these events.