Atari's Gem based TOS is my favourite vintage OS, specifically the post Rainbow variants. I'd probably edge 1.62 over 2.06 for compatibility sake, but regardless of the later version, TOS beat the hell out of the MAC or Amiga OSes that existed simultaneously (note, I'm not saying ST's were superior to MACs / Amiga's in every regard, but their OS had no era-specific competition).
OS9 was my primary MAC OS, and I hated the introduction of OS X, not because it wasn't an exponential improvement (it was) but because it wasn't compatible with my PowerPC MAC. I went PC as a result (probably a dumb move, but you know what they say about hindsight!)
From playing around with my friends' MACs though I'd say the Snow Leopard iteration of OS X has been my favourite so far.
For Windows based PC's Win7 didn't suck (although it did get rid of truly functional multiple toolbars - restricting them for the first time to the taskbar), but no Windows OS came anywhere near the customizability (is that a word?) of Windows XP, - the pinnacle of Microsoft OS'es. I keep an old XP machine around, and I am repeatedly blown away by how much better (and tweakable) is was than anything MS has released since.
For Linux builds, my fav was Ubuntu, but it's been awhile since I've had the time to look at more recent builds, so while I list Ubuntu as my favourite, ignorance to newer products probably influence that preference.
Worst OSes? Win 10 - no competition...it broke more than any other OS has fixed. (Latest victim, my left ALT key died with the anniversary update - which sucks for a bilingual typist) it slowed emulators, or made them unuseable. And let's face it, it really is just glorified spyware. Not since SONY planted rootkits on their music CD's has any corporation so blatantly insinuated themselves into their users activities, and so unapologetically invaded their privacy. It also removed any level of customization, making it the absolute worst OS for computer enthusiasts. (hell, the selectable colour palette rivals 8-bit computing!
Microsoft says that Win 10 is their last OS, and they will simply patch upgrade to newer versions from this point forward (à la OS X, how original!). I say that is a decision that will see the once mighty Microsoft go the way of AOL within two decades (if not sooner.)
But STe era TOS will always be the closest OS to my heart, with OS9 and WinXP bringing up the rear!