What is your budget?
The most capable cpu out today is the Intel i7-980X with 6 cores @3.33 and hyperthreading, giving 12 threads. If your application is multi thread enabled, that is the best, excluding workstations. It will cost about $1100.
A top cpu would be the i7-930 @2.8 for <$300. ($199 @ microcenter). With 4 cores and hyperthreading(8 threads), it can do a lot of work. It can easily be overclocked to 3.3 and beyond. On a clock for clock basis, the nehalem cpu architecture is a bit faster than the AMD offerings.
On a value basis, lesser offerings give you a fair performance for the dollar. The choice will be based on other considerations.