Paymentech. It's a hassle to get started but it's worth it. It's also a lot cheaper than Stripe and you don't have to worry about random account closures.
I'm looking for backup providers as well. I absolutely hate how you have to go to complain on HN (and have the post noticed) to get any support at all from Stripe. What is the hassle involved with Paymentech? How easy is it to integrate like Stripe is? How is it cheaper?