I visited Siem Reap, Cambodia on 21-24 November 2016.
In general, I suggest you to order a tuk tuk driver (15-20 USD a day) for a tour around siem reap and/or a guide at the counter where you buy the temple ticket. Ordering a private car cost plenty and sometimes tuk tuk will wait nearer the attraction unless there is a strictly designated parking spot.
Meanwhile having a guide has more advantages, he will tell you stories of the places or thing around, it’s better than to go around alone and are clueless about the meaning of certain things there. And he could even offer to take your picture instead of you relying on a selfie stick or tripod. Because sometimes there’s a certain place you can’t use your tripod and it also takes time to pack and unpack your tripod.
The temples are nice but some temples are similar with each other even though they have different names. Some temples are worth visiting because of their locations instead of the temple itself, for example it is on the hill, through the swamp, or in the forest, so you get relaxation on your way to the temple, seeing the nature.
My favorite temple is Angkor Wat and Bayon for the temple itself. And Neak Pean and Banteay Srei for the location.
Note: No disclaimer since the picture is mine. At Bayon Temple.