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The most common is Ubuntu since there is community support, and easy to install, also meets your requirement, they have upgrades but ofcourse even those who are familiar with linux always do backups when worse happen they have something to restore, you are responsible on what you run on linux or ubuntu, it won't run it unless you say so, even the firewall, 5 years is already long enough, what are you trying to achieve? security updates or application updates? I've been using ubuntu for years now I'm still have one running ubuntu 16, also someone using slackware though it has different purpose and still running for almost 10yrs or more now, without issue.
OP already said he don't like Ubuntu though. And how beginner-friendly is Slackware compared with Ubuntu or Linux mint? After all, their beginner guide (
) heavily involve terminal usage.