What is the best and proper tech stack to build a good recommendation engine?

I want to use MongoDB, but I can see that it's not a great fit for graph data. So, what is a good DB/stack to build a well performant recommendation engine which can be used in social network kind of apps? Some people suggest Redis + Neo4j but I am not sure if that's the best solution. Some also suggest Cassandra but again I have never used that in any previous apps. What do you guys think?

Thanks!