Are you talking adult or non-adult? If adult, you are limited to ccbill, ibill, verotel, jettis and epoch. All of these are good choices, some prefer one over the other but pretty much they are all first rate. If you have your own merchant account, some of these will process for you and there is also netbilling which is known as a top notch processor in the adult biz.

If you are talking non-adult, there is a lot of processors, I personally use worldpay.com and have always found them professional, reliable and very helpful.

cheers,
Luke