Best is too difficult to quantify without parameters that matter. Stripe is almost certainly the easiest to integrate quickly, but is definitely not the cheapest. Paypal has perhaps the broadest reach to potential customers, but doesn't work for certain payment types. Braintree is perhaps the cheapest at scale, but is more complicated to set up and has monthly minimums, making it ill-suited for bursty vendors.

They're all good, and they're all likely to meet a given need, but it would be easier to give a better answer if you list your criteria. Otherwise, you might have better luck on Quora (or here, maybe?)