If you define "best" as "cheapest" then the cheapest prices for domain names on the web is bound to be the monster auction house, eBay. You can sometimes find dot-infos there for a penny and dot-coms, nets and orgs for less than a dollar. The selection may be limited, the registration period remaining far less than a year, and your chances of finding the name you really need slight, but if price is the only determinate, by all means go to eBay.
Buying a domain on eBay will almost certainly require a PayPal account and most of the buying and selling on eBay is done by domainers. Domainers being those folks who, like land developers buying undeveloped land, buying domains hiring to someday sell them at a reasonable profit to people who want to put up websites on them. As you may imagine they are not selling their best domains at these fire-sale prices.
Price, of course is the principal reason to buy a domain on eBay. But be careful. Many domainers try until the last minute to sell their domain to higher bidders at different auction homes that specialize in domains. The domain you buy for almost nothing on eBay may be worth exactly because because it is fated to expire within days of your purchase and you will find yourself having to come up with the price of another year's registration before you can get any use out of your new domain.
For non-domainers buying cheap domains on eBay can be profitable, entertaining and educational. Keep in mind, however, that the people you are trading with are just as tight with a dollar as you are and most of them have probably been at it longer. So be careful.