What is your favorite payment processors that you use on your websites?
I personally like paypal a lot. It is so easy to integrate on your site and their protection against fraud is highly effective, although not perfect. I do get chargebacks from paypal once in a while (they charge $25/chargeback) but the rate is relatively low compared to those that I get from 2Checkout or authorize.net.
I give a D to 2Checkout. They are such a pain to deal with. They keep changing stuff in their system and breaking websites without informing their sellers. Whenever you call their tech support, they always argue that it is your fault that your site isn't working and offer very little support.
Authorize.net is ok but you have to do the fraud prevention yourself. Authorize.net is just a payment gateway and not a payment processor which offer some fraud protection. With Authorize.net, you are on your own and you must implement or turn on your own security checks.