For true data interoperability, as opposed to the silo data models required for property graph databases, the RDF w3c standard with SPARQL [2] is the industry standard. The new global API made possible with the Solid project [3] by Tim Berners-lee uses RDF
[1] https://blazegraph.com/ [2] https://en.wikipedia.org/wiki/SPARQL and https://www.w3.org/TR/sparql11-query/ [3] https://solidproject.org/
With respect to graph databases, the industry standard is Tinkerpop-Gremlin. Of the top 5 graph databases 4 support it, coincidentally the only one not to support it is Virtuoso. It's not like any other query language I've used, there's quite a bit of learning involved and the docs[1] sometimes lack sufficient detail, there's multiple ways of achieving the same result and unfortunately, some implementations don't provide the full language (eg Cosmos Graph [2] doesn't support the Match step or lambdas). But slowly with handy resources like (Practical Gremlin)[3] and trial and error, you can do some very sophisticated things.
[0]: https://db-engines.com/en/ranking/graph+dbms [1]: https://tinkerpop.apache.org/docs/3.5.1/reference/ [2]: https://docs.microsoft.com/en-us/azure/cosmos-db/gremlin-sup... [3]: http://www.kelvinlawrence.net/book/PracticalGremlin.html
SPARQL and RDF are powering the new decentralized web with a global interoperable api in the form of Solid (by the inventor of the web) - you'll likely be hearing a lot more about this as it rolls out commercially in the coming months