Does anybody remember Minix?
Around 1981, working with PDP-11 (Q-Bus) systems, RT11, TSX,
I encountered Integrayed Solutions Inc (ISI) Unix systems, 68010
and then worked with SCO Xenix on PDP-11 and PC systems.
Minix was an 8086 or 8088 teaching O/S. 8086 small model. ie.
64Kb programs and data. When Linus Travalias ported this to the 80386 in small model,
small model being 4Gb, The whole Open Source OS scene opened up!!!
My first distribution was OpenSUSE. Then I switched to Arch Linux, then to Fedora. 
Hello. Mine was Slackware, eons ago. I tried it because I read somewhere that DOOM played faster than in DOS. And it did!
I remember playing Unreal Tournament under Linux about 20 years back. So definitely performance wise much better than under Windows XP
Hi
My first distro was fedora, it was very nice about 2001 year
Slackware pre 1.0 (pre 1.0 Kernel) on a 486
UNIX, ATT 3B1(with Internet/SLIRP/UUCP)
Leading up to UNIX
OS-9(not really UNIX) on a TRS-80/CoCo
Vax/VMS
MITS/Altair(1st exposure to MicroSoft via BASIC)
Today primary Rocky9, used Debian, Ubuntu, Slackware,Fedora, RHEL, CentOS, Redhat Pre-RHEL
Other Work related
SGI/Irix
BSD
Solaris/SunOS
Data General/ Aviion
Harris Night Hawk
DEC/ ULTRIX
Great post, dear Ian!
My first distro was Slackware circa 1997/98. I must have run it exclusively until I switched to debian around the year 2005. I had used debian as my daily driver off/on (because I was/am a mac user also) until 2019 ish? Where I switched to fedora and later on fedora & rocky
As per systems I have used as a systems admin,
windows (obv)
HP UX
AIX
FreeBSD
CentOS
RedHat
Debian
And now I’m hoping we can migrate our aging centos servers to rocky <3
1990 - Prime Computer EXL 300 – running AT&T System V.4. The concept of a “distro” was not concieved yet. Also BSD Unix running on MIPS R3000 RISC systems (shortly before it became an SGI subsidiary). These were the days when “grep” seemed like magic. Then in 1993, SunOS, then Solaris, then RedHat and Fedora. Present day, CentOS moving to ROCKY!
Some time ago I described my journey from a Commodore VC-20 back in 1983 up to Rocky Linux on my workstation in a blog post:
My first distro was SLS, in 1993 I think, when I was running a BBS for the iCE art group, called The Pantheon. I switched to Slackware pretty quickly afterwords when I found I could run the BBS in a DOS emulator. We had a grand plan to bring ANSi art to Linux 
Switched to Redhat in 1996, and then Fedora which I still run to this day.