What os is best for my MacBook Pro Retina 13-inch, Mid 2014?
"Best" is subjective, but given that your Mac supports macOS Mojave, that would be recommended as it's the most current version (unless you have a specific reason not to install it, like a mission-critical app that you know isn't compatible with it, though that's unlikely).

Additionally, Apple generally only releases security updates for the current version of macOS and the preceding major version. So, when the successor to macOS Mojave is released, presumably around a year from now, High Sierra will likely stop receiving security updates and you'll want to move to Mojave at that point anyway.