Does not compute.

In one of the earlier versions of OSX, the chess game was a massive cheater. Whenever you would attempt any move that would take a computer piece, it would say that it was an invalid move and not let you.

+1 for acknowledging the existence of FreeBSD in your poll.

can't really say there is a best thing for me, its the overhaull of how windows act and function that made me like it, mac is not enough "malleable" for basic users as windows for me, and i found linux totally not userfriendly, and troublesome to manage when there is a problem. Though it probably became better in the last few years..

Amiga OS

BonziBUDDY

Gentoo

Dos 3.00 - Dos 6.22:
I was able to multitask:(I can format my floppy and use edit and play game at the same time by TSR programs.)
I was able to close windows when ever I want and start it when ever I need...
I can truly manage memory and other resources for gaming and other stuff. (EMM,QEMM,HiMEM,Smartdrive and etc...)
I got the feeling that the computer was mine and I am in control. (I know exactly what the machine does at any given time. Not like today.)
Now I am not sure what is going on and what will happen next....
God I am an old fart...

Steam?

Autohotkey for Windows :D