It really does depend on what you personally consider the best. Some folks are primarily looking for micro-niche stuff. Some prefer CCBill over NATS, and vice versa. If you are looking to fill a specific niche, what may be widely considered a crappy program by most may be pure gold for you.
Anyhoo, here's a list of what I consider to be the
best adult affiliate programs. To make it on the list, the programs must update regularly, be of overall high quality, and provide enticing and/or innovative promotional tools.