Revolut is great, but more like a banking servive than a payment service. You'll get virtual credit card details that you can use to pay, just like with regular credit cards. Haven't used N26, but I think they're similar from what I've heard..
Klarna is widely used, but I heard they can be quite predatory and make it very easy to make debt, which they then use to squeeze hella fees from you until you paid everything off.
No idea about Bunq.
Sync is not open source tho, just in case someone cares about that.