Depends on your requirements Mangastreamer, and where you are located. We've gone for the HSBC payment system because there is a strong HSBC presence in the UK, and it uses SSL for extra security (and it was the cheapest, relatively speaking)
You can't rally go far wrong with PayPal payments standard to be honest, unless it's not popular in your country (I know it isn't well known in some areas of the world).
If you need extra security, any of the payment systems which require SSL will do the job. Prices for signing up to these systems can vary greatly however, so do some googling of each one first.