First lets not start a war here, I am not trying to start a flame war etc...
Ok so there are 3 majors here, Windows, OSX and Linux.
I have played with all of them and feel I can make a good set of points :) Remember this is all opinionated!
1 Disadvantages of each system.
Windows - Very virus prone, very easy to exploit without the correct setup, It can be unstable, It has crap error reporting, I got bored of it as I use it daily, I dont learn much on it.
OSX - Too locked down via App Store, Expensive entry fee without Hackintosh, Pisses me off I cant replicate its looks perfectly, Too basic for me as again I dont learn much outside of terminal.
Linux - Dont have windows apps without Wine or KVM, can be a little tricky to setup, No full driver support (Ah well) Cant play all my games native.... Yet....
Okay they are my personal gripes with each OS but here are my advantages
Windows - Can play my games, I know it very very well, Almost every app I need is there, superb driver support, strangely at times Im glad of BSOD, easy to use (only because I have been on it since W95)
OSX - Stupidly easy to use which I like at times when I cant be arsed at all, Bit more defensive to viruses, Love the creative side with Final Cut, I love how it looks, The hardware looks superb.
Linux - Very secure out of the box, I have control over my system in anyway, No licences, No spying, FOSS!, makes me think in different ways via terminal, Can be easier than Windows once your familiar with it.
So a few personal ones there, I like each OS in their own respects, Im no hardcore user in anyway, and am no super pro with them, I know my way around windows enough to fix it when it breaks and how to solve problems on it, I know my way around OSX enough to help others, I cant do advanced repairs like windows but its an OS that doesnt really break :)
I know about Linux but not used to the filesystem or terminal etc, but I hope to do so soon, I cant do a lot of repairs on here but I know it well enough to install it and get it running.
So which is the best... Well if you have red this and taken it in, you will know none are, this is logical amongst a lot of people but some people still ask it, its all on your situation, Gaming is still king on Windows, OSX is still a power house for content creation via Final Cut X and its eco system with iProducts and Linux is still stable free and can be a monster in the correct hands.
So before asking, which is the best OS ever? try different ones, Get a virtual machine and run them, experiment and ask your self this question each and every time.
Personally I will more than likely keep switching to see what I like the most as my moods change, but never ask the question which is the best OS? without thinking of what shit you will cause via some communities and biased opinions.
Experiment is the only real way forward in this situation, but some people will never think like that they are too invested in the OS (I wont move from android because of this).
Hope I put some light on here.
May do stuff like applications etc also, I kinda like this series :)