I'll say that GraphDB is the best option out there, but I do work for Graphwise. (My personal blog shows that I was a big fan before I joined them.) You can register for a hosted sandbox version and also download and install it with a free license from https://graphwise.ai/components/graphdb/. See more at https://graphwise.ai/experience-area/.
New blog entry: Let's stop saying "semantic web" — like a startup pivot, the technology turned out to be great for things other than a new kind of "web". https://www.bobdc.com/blog/stopsemanticweb/ (In fact, "knowledge graphs" are an important part of the update of this thinking.)
For a Graph DB, I suggest GraphDB! (Full disclosure: I work for Graphwise, the company that makes it.) It's not open source but it's easy to get a free license. Please contact me if there are any questions I can answer.
New blog entry about querying RDF knowledge graphs: Selecting all the triples from all the graphs — but the default graph? https://www.bobdc.com/blog/selectingall/
