Best processor

The higher the processor speed, the better. In terms of overall efficiency, it depends on the CPU. For AMD Phenom II X6s for example it's best to run them at 3.4 GHz but with Core i7 it's better to run them at 3.66 GHz or so, for maximum efficiency based on what I've read.

In terms of type, Intel's current architecture is faster than AMDs, and the very best CPU for desktops right now is the Intel Core i7 980X.
 

ulysses35

Distinguished
CPU Mhz is no longer the best way to judge CPU performance. Intel ditched the old "Netburst" architecture because they realised the higher Mhz didnt not mean best performance.

The current leader in CPU architecture is Intel's i7 range of CPU's (9** series), for example the i7 920 = 4 x phyiscal cores & 4 x logical cores (HT) means Windwos sees effectively 8 CPU's. But its the design and effificeny that brings the performcne increase over the older C2D range.