I've been a bit of an OS whore over the years -- be it every flavor of linux distro (where they all suck equally as desktop OS), attempts at cloning windows like ReactOS, or other flavors of Unix and *nix likes such as Darwin, FreeBSD, QNX, etc.
But I'm more interested in the real "fringe" OS that few people have heard of and fewer still ever used.
In my case the favorite is BeOS, and by extension Haiku. If Haiku was more mature, had a better choice of browsers, and had a better selection of applications, it would be my go to. It feels so snappy, responsive... even the original BeOS was ridiculuosly well performing, back 20 years ago it could make a K6-2/450 feel like a bleeding edge hardware machine of today in how the UI performed and responded to input, or how well its pervasive multi-threading forced "give the user something" to the foreground. It was ridiculous how it could take 1990's hardware and handle things like multiple DVD quality video stream playbacks side-by-side on hardware where just one video would bring the machine to its knees in Windows and Linux... much less the realtime audio capabilities that make a BeBox as iconic a part of many recording studios as the Atari ST.
Sadly, I think said "pervasiveness" of multithreading, where EVERY program requires at LEAST two threads (input and display) is what makes there be so little software written for it. It takes an entirely different mindset and architectural design approach to write even a "hello world" for it, in a way that's so radically different from nearly every other GUI based OS.
But anyone have anything they love that's even more obscure?