For me, it was Gentoo. I was a little bit lost after a bad time in the public school system, and not sure what to do with my life after that. I was fascinated by Linux, but never really got the hang of it. Used SuSE, which was fine, but I never really knew what I was doing. And then I get into Gentoo. Messed up some stage3 installs until it all started to click for me and I had the correct mental models for stuff and how it all fit together to form a complete system. That basically kickstarted my career as a Sysadmin, and I'm still a Gentoo user to this day (but only on the desktop, too much compiling for my laptop...). I feel very, very grateful to all the folks involved. Of course to all the people behind Linux, but Gentoo especially. In a way, it saved my live. Not sure where I would have landed w/o Linux.

I've also used Gentoo professionally on many occasions, of course for one-off servers that I treated like pets early in my career, but also in a more cattle-like role. Had a custom updater that copied a new stage4 into a "B-Slot" (btrfs subvolume) from a golden master system. That contraption drove about ~25 POS systems for a small retail chain.