Which database backend should I choose for ASP.NET Core for running on Linux?

At the moment I'm learning ASP.NET Core and the main thing I'm struggeling with is the database backend.

My question is, which open source backend would you recommend for developing with Visual Studio 2017 and .Net Core 1.1 MVC?

I've looked into several tutorials for database backend and EF + a couple for MongoDB, but most of the tutorials are rather dated (if November 2016 can be seen as dated).