trusteft: To answer your question, I meant TOS as in The Operating System, the OS for the Atari ST line of computers.
Yes I know of Haiku, but...
timppu: I've never used Atari ST because I had an Amiga 500 instead...
So what is so great about TOS? When it comes to Amiga's Kickstart/Workbench, yeah it was nice for a late 80s OS, but oh well, we have moved on...
I had Amiga too.
What's great about TOS? Hmm, well let's see. OBVIOUSLY it's not a modern system so it couldn't stand a chance in today's world, but it was very simple, very light, all in ROM, 100% plug and play, and as far away from bloated as possibly while remaining effective in doing its job. You didn't need to spend time fighting or even working on the OS. As an end user you just used it to run the software you wanted to run. You didn't have to change anything (apart from changing resolution when needed, or enabling/disabling the blitter or later on changing the CPU speed. Depending on the model of course.
Amiga's OS, including Workbench was technically more advanced in terms of complexity which gave it advantages (like earlier multitasking), but also disadvantages like (if you wanted to use the Workbench) much larger memory footprint, more or less requirement for a HDD for later versions (3.x), etc.
As an owner of both series of systems back in the day, they were both good for what they did.
I just happen to prefer the simplicity and aesthetics of TOS.