You have to be more specific. Like specify price, utility, etc.
Anyway, the best desktop processor money can buy today is the Intel i7-980X. 6 cores, Hyper Threaded with 12 MB L3 Cache.
Intel also has a server processor with 8 HT enabled cores.
The fastest AMD processor is the Phenom X4 96 BE. operates at 3.4 GHz. AMD is also coming out with a 6-core processor sometime in June.
^+1 but you need to be more specific like the other guys said and don't go buying an intel i7 980x because you could just get an intel i7 930 and overclock it to 4ghz and it will probably perform just as well in all the tasks you will be doing. And it's cheaper.
I'm going to answer that good question in an
Let's say that you have a lot of data that you
need to backup on a daily basis, for the sake of
security and posterity.
And, let's say that you have an older motherboard
and matching CPU, say something with the Intel 875 chipset
and a 3.2 GHz Prescott CPU.
The number and type of hardware instructions that are
needed to perform routine backups is a small subset
of what that CPU can do.
When wired to a Gigabit LAN (Local Area Network)
that chipset and CPU will work fine for this purpose.
Similarly, because an I/O-bound task really doesn't
need a super fast CPU, you could build such a "backup server"
using a very low-end Celeron-class CPU, even one with a
single core (remember those days?)
So, the question needs to be modified to ask:
"Which is the best processor for any given task?"
The i7-980x is arguably THE WORST processor
for our backup server's tasks, because it will be
a huge waste of money that could be spent
instead purchasing a capable PCI-Express
system with modern RAID controller and
matching storage devices.