The best operating system for a given user is the one that causes the least number of problems for that user. If we define “problem” as a difficulty that is not easily solved without technical skills beyond those of the average user, that excludes the geeks who just love having a puzzle to solve and bitch about later.
Easily 90% of users just want to turn the machine on and check their email, surf to Amazon or their favorite porn site, and so forth. If they want to do homework or make up a flyer for the quilting society, they want to do it in a word processing program they are familiar with, without messing around with complications of any kind. People like the familiar. I have a family member who once ran a nuclear power plant who won’t switch from AOL because he simply doesn’t want to take the trouble to learn a new system.
If these users started on Windows, the best system for them is Windows; if Mac, then Mac. They don’t want the hassles, even the little ones, of changing the way they do things on the computer, which to them is like the TV — turn it on, and it’s supposed to do what you want, and they couldn’t care less about the magic inside. I may well switch to Mac with my next machine, or to Linux, but I will not recommend either change to most people for the above reasons. Linux, in particular, will not become mainstream, in my opinion, until applications like Word, PhotoShop and T-Shirt Printing for Idiots work seamlessly out of the box. This will involve developers getting together on a distro, and working on it for maximum compatibility with computing’s legacy programs — the ones most of the world uses.
Until then — until all the fan-boys and -girls get their heads out of the sand and go for compatibility instead of parity — the best operating system for most users will remain Windows, whether we like it or not.