You never forget your first UNIX.
Slackware Linux.
Prior to that, I had used only PCs and Macs, and "big" (relative term)
systems, like PDP-11, 3090, 4361, and VAX. Linux opened up a whole new
world for me. From there, I made my way to everything else (Sparc Solaris,
AIX, HPUX).
Lots of fun. Thanks Slackware...
- Earl