Definitely Offervault. It's sort of a search engine for various CPA networks rather than a network itself. Think of it as KAYAK (the travel site) for CPA networks. I don't use it to find networks as much as I use it to find hot campaigns across the networks I'm a member of.
They also have weekly webinars that are occasionally useful. They're usually a sales pitch for something, but they sometimes have nuggets of wisdom that prove to be useful.