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.