This is the best one! Absolutely amazing and just a few other tourists. But don't go if you are not able-bodied. Some areas are really hard to get to.

If you can't make it there (it took us about two hours by car to get there) definitely do Ta Phrom, the cleaned up 'jungle temple' version. And, as GadgetFreak said, don't forget to walk all the way through.

Kbal Spean is also nice. Not a lot to it, but combined with the little hike you need to do and the whole athmosphere it was pretty cool.

Bantey Srei is beautiful but fully restored and often fairly busy.

We stayed five days and toured the temples on three of them. On the other two, we toured the spas.

If you have only one day, do Beng Mealea and Kbal Spean and maybe Bantey Srei. If you have more time, also do Ta Phrom, Bayon, and the Angkor Wat.