"Best" is a strong word to use universally, but IMO when in doubt use PostgreSQL. It has a very solid traditional ACID/SQL core and it has also been introducing "NoSQL" style json document storage on top of that for projects that want to go that route. I suspect PostgreSQL will do the the NoSQL type stuff well a a lot sooner than NoSQL databases will do the consistence/durability thing well.

Having said all of that on my personal site I actually use RethinkDB because it is delightfully easy to setup and code against (I just persist my Go structs to the db and don't worry about a relational schema to maintain) and I don't really care about data integrity there, so I don't mind that RethinkDB is relatively new and untested. So one size doesn't fit all, but PostgreSQL is still, IMO, generally the "safest bet" if you do care about data integrity and don't want to spend hundreds of thousands of dollars on licensing to scale it up.