>>1034920 (OP)
for microsoft window operating systems:
Windows XP (offline only to avoid malware, but you should be fine using it here and there with a firewall)
Windows 7 (pretty modern window operating system that isn't spyware ridden as 10 and is pretty stable, but support will end in 2020 and will end up like XP)
for gnu linux operating systems:
Slackware (tries to closely be UNIX like which makes this a 'pure' operating system; but you will have to get used to using ldd, make make install, or slackbuilds)
Gentoo (your (or somewhat) entire system will be compiled by your chosen CFLAGS and USE flags; this is helpful if you want to avoid certain packages from being installed due to dependencies (librsrv pulling in rust; firefox pulling in pulseaudio, etc))
containing systemd (because some people like it and some don't):
Gentoo (using a systemd stage3)
Ubuntu Xenial LTS (to me at least, isnt as bloated as the newer ones and is certified to Just Works(TM))
Arch Linux (its ok i guess)
for bsd (or bsd based) operating systems:
FreeBSD (seems to be lacking drivers for some devices, so be sure your device is supported)
OpenBSD (secure by default ;) )
NetBSD (if you want to run BSD on some old device)
DragonflyBSD (not too sure about it, but to me it seems to strive for performance)
Mac OS X (i like the style and the way it is, but it is like Windows XP; you will also need a powerful pc and hacks to get it to work)
macOS (sometimes is buggy and annoying, but is a good alternative for someone to switch to from Windows to get into unix and unix like systems a bit easier; you also need powerful pc and hacks)