Where is the best place to put your pay per click advertising dollar?

The only pay-per-click advertising I’ve ever paid for was through Adbrite. What I don’t like is that their cost per click is just an estimate and is not necessarily true. Other alternative I’ve been considering were Adsense, Search Advertising, and Kanoodle. What do you think is the best pay-per-click advertising network to invest in and why?

Original post by gregdavidson