DID any hacked Mac operating system
And what is the best OS in the world

Mac OS X is safe because, by default, applications cannot be installed without authentication by the user.If you feel really paranoid,just disable java in your web browser.

As to The best OS in the world.That is a matter of choice.I use linux as my main OS,because I do not have to worry about installing anti-virus software (you can install clamwin).Or worry about anti-spyware software,adware,or a defragmenter,as linux has no registry.A Firewall is installed in all Linux distro's by default,called iptables.I do have a reserve PC using Windows,and all the above,except clamwin,I use (Avira anti-virus) ,are installed.
It is all personal choice for the Home user,and what you intend to do.If you are a huge "gamer",and do not have a game machine (XBox,PS3,etc),then a high-end Windows machine would be ideal.At the moment,and I will probably get "flamed" for this,it is the better option than a Linux distro,due to configuration problems,which can arise,and lack of choice by the big gameing companies.Although there are "good" games" to be bought,which will play on Linux.And with some effort,you can get an iPod to work in Linux,although I know friends who have had a few headaches at the attempt.

Linux is the best mate