MariaDB

I recommend MariaDB. For 3 main reasons: Stronger security, Easy drop-in replacement, and increasingly more users

Details and sources below

Stronger Security
MariaDB is more secure than MySQL. Because MariaDB source code is fully open source. For those not familiar with "open source", it means that MariaDB is more secure. Because its code is fully publicly available for review and contributions. The more people are able to review the code the more secure the code is.
• Source 1 at https://en.wikipedia.org/wiki/MariaDB
• Source 2 at https://mariadb.org/about/

Compare to MySQL source code which is not fully open source. Only part of its code is publicly available for review and contributions. In other words, less people able to review the code means less secure. The other part of its code is not open source. It's proprietary. Meaning you have to pay to use it.
• Source 1 at https://en.wikipedia.org/wiki/MySQL
• Source 2 at https://www.mysql.com/downloads/
• Alternative source 2 at https://archive.fo/TvaPZ

MariaDB is owned by the not-for-profit MariaDB Foundation. Often the first priority of not-for-profit organization is to serve you and protect your privacy and data.
• Source at https://twitter.com/mariadbfdn
Compare to MySQL which is owned by the for-profit Oracle Corporation. Often corporation first priority is to hoard money for their anonymous share holders, not serve you and not protect your privacy and data.
• Source at https://en.wikipedia.org/wiki/Oracle_Corporation

MariaDB is guaranteed to stay free and open source.
• Source 1 at https://mariadb.org/about/
• Alternative source 1 at https://archive.fo/CW4sw

Compare to MySQL which is not guaranteed to stay free and open source. Which means that without notice tomorrow morning, the Oracle for-profit corporation could ask you to pay to continue using MySQL.

Easy Drop-in Replacement
MariaDB is a drop-in replacement for MySQL. This means it's usually relatively easy, quick, and cheap to migrate from MySQL to MariaDB.
• Source at https://mariadb.com/kb/en/library/mariadb-vs-mysql-compatibility/
• Source alternative at https://archive.fo/R7FJZ

Increasingly More Users
MariaDB is use and sponsor by multiple well known and large organizations. Including but not limited to Google, Microsoft, IBM, Mozilla, the Wikimedia Foundation, DBS Bank, Debian, Archlinux, RedHat, Fedora, and CentOS.
• Source 1 at https://en.wikipedia.org/wiki/MariaDB#Prominent_users
• Source 2 at https://mariadb.org/about/supporters/

Compare to MySQL usage and sponsors which are declining to the benefit of MariaDB. Mostly due to the benefits with MariaDB. Which are listed above. This is based on my personal experience as Senior Product Manager at Ubertus.org and most of our clients. Disclaimer: My opinions, suggestion, or recommendation expressed above are solely my own and do not express the views of others people or other organizations. To each their own :)

I would not touch it. The download server runs with different "security" settings and is still outdated and dangerous.

https://www.ssllabs.com/ssltest/analyze.html?d=downloads.mariadb.org