If safety's what you're after then Open BSD can't be beaten, although I doubt it's up most users streets.
Mac OS X is probably the safest of the 'mainstream' OS's, but I've never been much of a Mac fan.
My main OS's are; Ubuntu, Free BSD, and Microsoft XP.
Mac's are overpriced. Linux Ubuntu is free.
Have found Ubuntu to be quite useful.
Put it on a few older low end PC's where all of the service pack updates, virus, spyware and firewall programs had left the XP machines virtually unusable.
For people using their machines for basic web surfing and e-mail, Ubuntu works fine.
The need for any virus software goes away.
It is about bulletproof on security and a very stable OS.
It is not a great solution for users with high end applications.
However, I am seeing it more & more of the Linux desktop in the business world.