Depends what you use it for:
-Servers - Intel xeon...but the opteron is pretty good as well and it will save you some $$
-Workstations - By far the best for the $$ is the Amd A10 6800 - Decent on-die radeon card (not great for gaming)
-Gaming machine - AMD Black edition processors...More cache, they really do very well and you can get 8 cores pretty cheap
-Most of the Intel i5 or i7 processors are good, but will cost you $
-Intel is way better right now if power consumption is high on your list
-AMD's tend to have better overclocking with active liquid or TEC cooling, my buddy has an AMD black running stable at 6.2ghz