I've been using Unix in one form or another since the mid 80s, so that's pretty deeply ingrained by now.
I was strongly biased towards Solaris & OpenBSD for many years (Solaris on nice Sun hardware, OpenBSD on small machines) but both began to annoy me a little bit recently, so I switched to Void linux. (Also, there was ONE feature of Linux that I REALLY wanted - extended attributes (name=val) in the filesystem. Love those.)
I'm fascinated by Multics & Control Data's NOS (70s mainframe OS's), but that's for historic study, not actual use.