939 is the newest and will be around for the longest. Motherboards with 939 pins are arriving now with PCI express sockets. For a system that is to remain upgradeable at cutting edge level for longest it is the way to go
But at the moment it's expensive
754 will take a full range of processors now and AMD says it will remain upgradeable for some time. It is much more reasonably priced
The best answer for value may well be a 754 processor with Athlon 64 3400 Newcastle.
However, it has to be said that for most folk any of these will provide performance they are unlikely to use