SQL Server to MySQL, which fork?
MariaDB is the open source community driven version that has the  endorsement of all of the unaffiliated MySQL developers.
MySQL is a trademark owned by Oracle and which has continued developed funded by Oracle. Some of this development is pretty awesome. I wouldn't discard it. I believe the Oracle developers are likely more skilled than the community working on MySQL.
PostgreSQL is the best and most advanced open source database. Though it has nothing at all to do with MySQL, I highly suggest you use it instead. It's faster, more spec compliant, and better documented.


The Maria vs MySQL is a mess and all feature comparisons are pretty bad. So read them all! Good luck with it.


MariaDB official comparison: https://mariadb.com/kb/en/mariadb/mariadb-vs-mysql-features/