If you go to Laos, the Thakek loop is the best place to ride. The loop is only 3 or 4 days, but it was the best riding that I encountered in that country. The Bolaven Plateau out of Pakse is ok (and is a longer loop), but Thakek was more enjoyable, in my experience.
Thakek sounds good! I'm definitely going to the Mekong, then over to Cambodia, for the first week. Now that you've thrown Thakek in the mix, I'm considering Laos haha!