What "the most secure" is will depend on what you are trying to achieve and how you use it, but here is a nice list.

Some notable examples (according to myself):

Qubes OS - Provides security through isolation/sandboxing, among other means.

Subgraph - Debian based, recommended by Snowden, if that means anything.

Alpine Linux - Lean and clean, uses musl instead of glibc. But does not seem to be as sophisticated as the other two distros in terms of security measures.