The one that fits your need. If you keep the persistence layer properly separated, you can switch DBs as your needs evolve.

Recently I've seen a shift back to SQL, especially Postresql. Couchbase seems to be popular too.

I'm assuming "modern" means "bleeding edge" otherwise, whatever you've worked with in the past probably fits.

I see that a lot of people are interested in MongoDB, PostgreSQL, Apache Cassandra, InfluxDB.