I don’t know about best, as there are lots of trade offs between various projects. I’ve reviewed and worked with a lot of Java based ones, top of mind:

- Airflow

- jBPM

- Enhydra Shark

- Activiti

- Netflix Conductor

Most of those are more workflow engine than pure orchestration.

A few years back, we searched high and low and I was generally unhappy with the commercial offerings so we wound up building around Netflix Conductor (after a disastrous run with Joget, which is built on Enhydra Shark).

Since then I’ve been pretty happy with Conductor, submitted numerous PRs and accidentally became one of the people keeping the MySQL backend implementation going forward.