Most likely, affiliates are going to want an affiliate backend they are familar with. That means, pretty much, either CCBill, MPA-3, NATS, or Executive Stats.

Right now, my vote would probably be for Epoch's version of MPA-3, which is free, and does cascading billing. The only catch is that you have to use Epoch as one of your billers, and you must put Epoch first in the cascade.

If you want to wait a little while, CCBill's affiliate program is being significantly enhanced right now and supports cascading billing and will soon support many of the other features that NATS and MPA support.

NATS has had a bad rap recently because of some security breaches. It's feature-rich, and we and many other programs, including some very large ones, do also. Until the recent issues, and NATS non-transparent handling of those issues, it was pretty much the top-shelf choice in the industry. But if I were choosing today, I would very seriously look at the other choices in addition to NATS and weigh everything.