Define "best" Do you want the "best" gaming machine, "best" video machine..etc. There isn't a "best" place to buy parts for a computer. But a few really good sites are, newegg.com, pricegrabber.com, tigerdirect.com, and zipzoomfly.com.
Also this is just me being nitpicky, but a cpu is a component to a computer, it's not the computer itself, so if you're going to refer to a computer, please say computer, tower, or box.. people will get less confused when you say something along those lines rather than saying cpu...