This question is kind of... self explanatory. Go to benchmarking websites and find the one with the highest score... then you purchase said CPU. Consider that the highest score will likely be incredibly expensive.
Like what do you mean? If pure power is what you're looking at grab a new x99 motherboard and an i7 5960x. But both of those things alone plus the ddr4 ram you'll want with it will cost you around 2.5k at the least.