I have been to all three places, and would chose Luang Praban. We were there 2 years ago; it had been discovered by backpackers but not higher end tourists. That is quickly changing. In Siem Reap, it was horrifying to see all the huge hotels under construction. You want to visit Luang Praban while it is still in its natural state.

It's difficult to articulate why I find LP so magical. We looked at each other and laughed for joy when we left the airport in a tuk tuk down a dirt road. There's a lot to do and see, and it's special when it is still not developed for tourism.

Be sure to go to the Khmer Market. We went to the local Red Cross office for massage, about $3 an hour. And we rented a scooter from a shop boy a block or so from our hotel and used it for the day for just a few hours.