Arch Linux. Everyone said it was hard to use, unstable, etc. but my experience with it has been the exact opposite.
Yes, the install process is needlessly complicated (although it got a lot simpler now that we have archinstall), but the OS itself is rock solid and rarely has any issues that require more than a reboot or a package reinstall to solve. The AUR is a godsend too if you don't want or don't know how to compile stuff from source.
Arch Linux has by far the best community, the support wiki is the most useful wiki to Linux there is, it basically covers everything. Mad props to the arch Linux community.
I heard all the stability concerns when I first started using it. That was in 2008. It's been my main distro ever since. Apart from 2 or 3 major changes over the years (eg, the infamous /usr/lib migration) it's been rock solid and very up to date
I second this - for some reasons, my (almost) first distro was arch (first was a fedora for 3-4 days). Arch is great if you know what you are doing, you can have a lean mean compute machine