Mac OS is a variation of BSD that tends to run best on mac hardware, which tends to be more expensive, so I've never used it. Windows is the standard for operating systems, that supports the most hardware and software and is used by the most people (on desktops). As the saying goes, no one ever got fired for recommending windows. Linux is becoming ubiquitous on servers, but causes the most problems for desktop users. It's arguably cheaper to run, because you don't have the option of buying much software for it.
My personal opinion is that windows is best for game developers. You can run a type of linux on it, if you care about a minuscule market, and you're constantly testing the most common operating system (besides android). Mac is a strong contender for other developers. Linux should be used only by zealots and other madmen (like me).
Seriously, I recommend linux for old and/or non-technical folks who have a family geek to maintain it. Windows is easier to screw-up, so your geek will be busier cleaning junk off of it. My grandmother nearly fell victim to an anti-virus scam. I happened to visit her while the scammers were "cleaning" her computer, so I immediately unplugged it and called her bank to stop payment. I then installed linux on her system and told her to ignore any warnings of viruses she got or requests for money regarding her computer. She was perfectly happy with linux to the end of her days.