Google’s internal library Truth has been released as version 1.0. It maintained by the Guava people, and is designed to work for both Java and Android according to their page. (Though not sure exactly what the Android tweaks are.)
I like the API for Truth and AssertJ and their error messages. Honestly I’ve been using jUnit’s assertion library mostly because I’m lazy. Though I’m considering switching going forward. Not sure I’d convert all my old assertions at this point.