The difference between them is the chipset that they use. The A7M266 uses the AMD761, the A7N266(I guess it is a N instead of a B) uses the Nvidia Nforce chipset, and the A7V266 uses the VIA KT266 chipset.
But to choose between them really depends on what you gonna use it for, and how much you want to spend.
Best motherboard is the A7V266-E, in my opinion, this mobo uses the VIA KT266A chipset.
Tell us what are you going to do with this computer(office working, writting documents, surfing the web), so we can recommend you a good motherboard to use.
Raf