I’m biased towards java.
I feel c# concurrency is less impactful because most of the important concurrent systems use *nix servers not windows, and most concurrent programming jobs do not go to windows developers.
Outside windows, c++ concurrency is mostly based on the C library pthreads, non-OO and inconvenient compared to java/c#
The c++11 thread classes are the next generation following pthreads, but not widely used.
Java’s concurrency support is the most successful among languages, well-designed from the beginning and rather stable. It’s much simpler than c++11 thread classes, having only the Thread.java and Runnable.java data types. More than half the java interviews would ask threading, because java threading is understandable and usable by the average programmer, who can’t really understand c++ concurrency features.