What is the most recommended database engine? I will have many databases, many users, and few processes
Oh man, it's so general question ;) It's always depends from what kind of app you are developing, what are the main purposes of the application, there are so many questions and options before you choose which one you want... ;) I'm using most popular MySQL/MariaDB and MongoDB, but I like and used some time ago CouchDB/PouchDB set and RethinkDB (which is being actively developed again btw).
Probably your best bet is MariaDB - simply because it is the most common and will probably fit your needs. Most common = most help available.