+1I voted 8 (since that is what I'm using), but next version will be OpenJDK 11 (currently dabbling. No big $'s for Oracle here).
There is nothing to learn. Especially with non-ui projects. With UI projects you need to use the simple and powerful B4JPackager11.Still on Java 8 as I'm too busy to learn a new version at the moment
I don't think that there is any library not compatible with Java 8, 9 or 11.I'm at JDK v9.0.4 until my project Athena is completed. It's very compatible with many libraries in this forum..