What is the best open source java workflow framework (e.g. OSWorkflow, jBPM, XFlow etc.)?
I'll cast a vote for jBPM. We used it on a larg-ish ETL platform in-house and it seemed to work quite well. I don't have anything to compare it to, however.
Here's an article that compares kBPM, OpenWFE, and Enhydra Shark that looks like it has some good, thorough info.
It depends what kind of initial investment you want to make. jBPM is the best in terms of features and flexibility, but OSWorkflow is a more lightweight, easier to get up and running and has with a smaller learning curve.
Drools Flow is the best workflow solution that I came across recently. It has a luxury to be better than other solutions, since it is built and designed recently, and based on lessons learned from other long existing, somewhat over engineered frameworks.
Drools Flow comes as a community project along with an official Drools 5 release that besides Flow includes: Guvnor, Expert and Fusion.
Unfortunately Drools Flow does not have an official Red Hat support contract yet, and that is a stopper for some big corporations to consider it. One might think the support is not there for political reasons due to the jBPM project living under same support roof.