I have used several sites. To me Paypal is the best. Easy and quick and most people use it. I would say second is 2checkout and Plimus something like that cant rember.
I would rather conflict with most of them here, paypal is not the best and has ONLY gained good and reliable trust over the people who are looking out for buying stuff online, the buyers. Yes paypal is a buyer's paradise. While selling products on ebay, you are the buyer and have made your transaction through paypal, then you are the God. Paypal will do whatever you say and even refund you the money, if the seller is not able to show the appropriate proof required by the paypal team. And yes paypal is also a great tool for the spammers. I have had pretty bad as well as good experiences with paypal since last 10 years. So I would not completely agree that its a 100% best payment processor in this world.