Children, please.
The short answer is: use whichever OS suits you best.
The long answer is that you have four realistic choices, each with their pros and cons:
Windows Vista: needs up to date hardware but that can be obtained cheaply, restrictive license, generally nice to use, huge range of software
Windows XP: will run on older hardware but lacking in up-to-date features, quite restrictive licence, huge range of software
OSX: will only run on hugely overpriced hardware from a single vendor with attitudes to user freedom at least as nasty as Microsoft's, smaller range of software but much good quality stuff
Linux: the software's free, the licence is open, the range of software is huge but most of it isn't good quality for the end user, wide range of hardware choices