[amiga386]

There have been some very nice OSes in the past (e.g. AmigaOS, BeOS, QNX, Plan 9) that all failed to the Wintel juggernaut. MacOS only just made it out alive.

I wouldn't say Unix itself is the best. It suffered a war between competing implementations pushing their own proprietary components. POSIX is the compromise.

But, ultimately, what is good about it today is not so much "Unix" (the proprietary OS from Bell Labs and its heritage), but specifically Linux and the BSDs. Why? Because they are actually open. They are freedom incarnate. You can add anything you like to them, today, without asking any permission. Not just their kernels, but their userlands too (Linux obviously varies by distro here). There's even a chance you can get your changes adopted upstream (unless it's GNOME), much more than you'd ever get from a proprietary company's OS.

So, while there's always room for improvement on the technical aspects of the OS, the social and political aspects of Linux and the BSDs make them the best we can achieve as a society.