Plan 9

It is an absolutely revolutionary OS by some of the original creators of Unix, that extends its core concepts in more coherent and elegant ways into the world of modern computing, instead of having everything from networking on up be tacked on by people who were perfectly capable but lacked the vision.

Examples:

Instead of NAT, if one machine on your network has the internet and the others don’t, you can say “use that other machine’s network stack now” and boom everything works. Your machine knows what its real external IP address is, it can listen on world-facing ports on the other machine as it needs to, everything works and is simple.

There’s a command for “run the rest of this session’s commands on that other machine’s CPU / memory” and it all just works. The sensation is that your computer just got magically faster.

Etc etc. I actually haven’t played with it extensively, and deployment is so limited that I’m not sure how useful it would even be, but if you are a fan of well made OSs that do things in a genuinely different fashion, it is objectively the best option to play around with. sdf.org has a place you can get an account on their Plan 9 machines and they do little free beginner courses in it over livestream.

TempleOS, because for it to go mainstream, a sizeable chunk of the population would need to go fully insane, and I think that'd be interesting

ReactOS. The "We have Windows at home" OS.

Maybe then it will see proper development to become that which it should be.

I'd love for FreeBSD to become more mainstream/popular (again)

TempleOS

Haiku - based on BeOS

"inspired by" would be more accurate. there's no original BeOS code in Haiku for legal reasons (other than the interface, which was open-sourced with the release of BeOS 5). All backwards-compatibility with original BeOS software is (impressively) reverse-engineered. Haiku OS is, itself, original software made to - in every way - look, feel, and operate just like BeOS did.

edit: i had a buddy in high school who had a BeBox. it was like having the best of a Mac and a PC in one machine. it really was a spectacular machine and OS. i really wish Apple had picked it up, but they went with NeXTSTEP instead, which, i admit, was still a pretty solid choice.

PalmOS as an alternative to Android/ios

Redox isn't Linux, it uses its own kernel. I want this one to succeed above all others, just because Rust was born to perform this kind of application: guaranteed memory safety when dealing with tens of thousands of lines of code handling hundreds of moving parts running thousands of different tasks, all at a very low level.

I'll second Plan 9, just because it sounds like scifi and truly takes advantage of how interconnected all computing hardware has become.

Third place goes to anything based on GNU Hurd. The microkernel architecture intrigues me, and I'd like to know how it effects the end user. Plus I'm just a big fan of the copyleft/FOSS aspect.

Also, I'd just like any mobile device alternative that's not AOSP, and Linux seems like a bad fit for mobile in general. Why do we need a fully-featured, all-purpose kernel when we're only gonna put it on a known number of SoCs and therefore a known set of hardware configurations? We could be optimizing the hell out of our privacy-friendly mobile OSes, but instead we've shackled ourselves to google or linux

Firefox OS. ...really I just want Chrome OS but FOSS by Mozilla. I know it's anti-privacy, but having sign-in + 1 click deployment on a new device is dope

Haiku is already pretty great to use in my opinion, despite still being in beta; with the right hardware you could easily daily drive it

OpenBSD. Imagine everyone just running a secure OS.

I remember running BeOS back in the early 90s so I guess I’d go with Haiku

HURD, obviously.

I'm happy with any OS as long as it's GNU.

ReactOS would be kinda fun

Redox OS seems like it's turning into something cool.

Serenity for sure. I love the 90s aesthetic and would like to see it make a comeback. At the very least I'd like to see their Ladybird browser become mainstream - we really need more alternatives to the Chromium family.

To name something that hasn't been mentioned yet, ArcaOS, which based on OS/2. It supports modern hardware and in addition to some preinstalled software, it also has some compatibility layers to run software from other OSs.

Refox OS. I know today isnt a magic bullet but it makes committing memory mistakes a lot harder. Also rust gets first class status as the is standard library calls it and we can slowly get over the legacy of C.

Something with a microkernel.

Or ideally, something like Android was supposed to be, with exclusively non-native executables. "Java but good." So I guess some .NET atrocity, or an obscure SPIR-V project.

Serenity is beautiful, so I guess I would pick that.

The only one of these that I know is Haiku (as an extension of BeOS). I was already a Mac user when Apple was flirting with purchasing Be, so I installed in on my PowerMac 9500 and took it for a test spin. I liked it, though I was too young and inexperienced (and this was pre-broadband) to really get a good feel for it. I think I switched back to MacOS within a day just because what else did I know to do with it.

MorphOS. It's still kicking.

Nobody's mentioned Guix. It's a GNU project, which is like Nix, but has a number of novel features. I'll copy in from my own thread about it:

Based on what I’ve heard so far: GNU Shepard instead of systemd, a package manager that compiles things from source and allows user-defined compiler options, a totally different way of arranging system files, and Guile-Scheme is used for everything; it sounds like there’s no other kind of configuration anywhere.

It's planned to be Hurd compatible, so I'd argue it counts.

OS/2 Warp

Yeah, I was going to say Haiku. It's surprisingly capable for this day and age (although needs tonnnns of work)

Haiku, but honestly I'm just happy to see the conversations here

There are a few aspects we need to consider...

For the UI, I would go with beOS, Haiku is cool, but the look is a bit too modern, I love the clean look of beOS's UI

For the best compabillity, after a lot of work, ReactOS, a project aiming to create an open source os that is binary compatible with Windows 2000.