Just to compare apples to apples—by which I mean credit card payment APIs—Stripe's Java API is superior. I'd not say it's a thing of beauty, but then, I don't want a credit card payment API to be anything but immediately approachable and obvious. And it is precisely that. Easy to understand.
As for something I am happy with in recent memory, the Undertow builder model is appealing: http://undertow.io/
But you ask a tough question. I've not spent much time thinking about the best of the best. I suppose part of it is that a great API gets out of its own way and lets you get to work. So I suppose citing Stripe's first may have been the right thing.
https://stripe.com/docs/api/java
As for something I am happy with in recent memory, the Undertow builder model is appealing: http://undertow.io/
But you ask a tough question. I've not spent much time thinking about the best of the best. I suppose part of it is that a great API gets out of its own way and lets you get to work. So I suppose citing Stripe's first may have been the right thing.