I would disagree - Best OS ever was OS/2 by IBM. The lack of applications available is what finally made me switch.... but IBM had nailed what an OS should do.
It was Object Oriented. Meaning that they took the metaphor of the Desktop and treated it as a real Desktop. Everything on it was an "Object" and had properties. Everything had right-click properties that included everything OS X, and more. By right clicking any object I could choose to make it a "template", for example (what Apple calls a stationery pad).
You create a series of printer icons that each had the properties you might need for different jobs, from the same printer. You would then drag'n-drop a document to whatever printer icon was appropriate. So, if you had a printer icon setup for printing photos at 4x5 on glossy paper, then if you had a photo (or a bunch) you would drag it/them to the photo printer icon and it would bang out the photos. Didn't matter what application you used to create the images, and you didn't have to open any application - printing was handled entirely by the printer icon. I think you can do something similar with OS X but it's not nearly as easy as right clicking a printer icon and setting up all the print settings and saving as a new printer.
Every single element of the GUI was customizable. Each individual icon could have its own font, size, font colour. Each folder could be customized. Not just the background colour, but the title bar, the scroll bars, the thin line around the window, the buttons, the background colour behind the buttons, the text (colour and font) etc etc.... each window had something like 15 or 20 elements that could be tweaked. You could a bunch of windows, of each window could be unique. Depends on how much time you had on your hands. All these elements were changed by using D'nD.... you opened a font palette (you would create a number of font palettes, each one with whatever fonts you wanted in that collection) and dragged the font. You opened the colour palette (see font palette) and dragged colours. Good Heavens I wasted more time making my desktop pretty than doing work some days!!
And finally (though this is not a complete list!) The Work Place Folder was the single most useful element - and I dearly wish OS X had the equivalent. It is the one thing I miss most. A Work Place Folder was a special folder.... Anything you dragged into it became an shortcut (Alias). You couldn't actually 'move' or 'copy' anything to it. What you would do is drag all the applications, documents, folders, remote folders, printers, font palettes and colour palettes, etc etc that you would need for a project. You open the Work Place Folder, and then you open the applications, foldes, palettes - whatever - and start your work. When you wanted to start something else or go home for the day.... you closed the WP Folder and all the apps, folders, documents, etc in that folder would close too. When you re-opened the WP folder everything that had been open previously would re-open as well and put themselves where you had left them. You would set up a WP Folder for each project and as you opened and closed them your work space would be restored. What a great concept. No more fiddling with the Finder going to the left when I'm in Photoshop and then moving to the bottom when I'm in Pages...
Sigh.... Maybe it's time I installed OS/2 (now eComstation) in VMWare? Just to waste some more time customizing it....
Incidentally, Apple and IBM collaborated on, IIRC, something called Taligent - which was supposed to be a joint project on an Object Oriented OS. Some elements made their way into OS X, but IBM really pushed it hard.
So - that's my vote.