From the very beginning it was planned as an active trip that is using local transport and walking … lots of walking, therefore it was obvious for me that photo gear must be transported in a backpack. At that time I used LowePro Primus which holds sufficiently two bodies, two lenses, flash and has enough additional space for some accessories, water and personal belongings. It proved to be a right choice. As to gear itself, if you are an advanced amateur like me, I do recommend as follows: two bodies, two zoom lenses, flash with light bouncer and maybe an off-camera cord. And now „why” part. First of all you do not want in place like this to end up with body which broke regardless the reason and come back home only with memories. Forget about any service near by and by near I mean hundreds of kilometers. Secondly and more importantly environment in Angkor is very dusty in hot season so switching lenses will cause all that dust cover a sensor. Third and last is that having two bodies gives you a possibility to use second setup much faster. I did not have longer zoom at that time but only Canon 100mm which is an excellent lens but longer zoom would give much more possibilities. Mind that ruins are high and in some cases not easily accessible due to renovation so if you want to catch some details you need longer focal length.
Summarizing, short zoom like 17-40 and long zoom like 70-200 would be an ultimate solution.
You may also need a flash because during a day in full sunshine in ruins you will encounter places with high contrast where without flash you end up under or overexposed.
One advice not photography related, take a 1,5l water bag with neoprene thermal isolation for straw.