Security is the output of removing vulnerabilities and insecure configs
So, the real answer is: what's the minimal software you need and the most regularly updated.
So, my choice is Arch.
Yep, installation takes a little longer and needs more technical skills, but only install the bits you need (also learn a little more this way) and then updates are tiny and can be done as often as you're comfortable with.
Whatever you choose, it will break / die / be deleted or corrupted one day, so always backup your data separately than the OS (separate drive partitions can help) and you're done.