The answer is not so simple. Firstly, you hve to understand that when people are using one Operating system, they tend to rave about it and say it is the best....
(Read the next answer! "I use XP, my SON uses Vista - I don't use Vista.... I prefer XP". How can people say they prefer XP when they don't use Vista? because they "HEARD something?)
I did an experiment with a flashdrive once, my friend made a zip file 1.9GB at work - office files, pictures, all kinds of stuff - 441 files. We unzipped it to the XP desktop, and it took 5 minutes - not bad right?
With his new computer running Vista Ultimate with a 2.6GHz core2duo - it took an incredible 14 minutes!!! and on my ubuntu machine, it took only 8 seconds.
I bought a Vista machine, and it has some lovely features (all of which are available in Ubuntu linux for free) - but inevitably, Vista does suck - some things take sooo long. Sometimes, you try to delete a shortcut from desktop and you get a dialog saying 'calculating time required to delete shortcut'.
wow, sure the WOW starts at that point! Delete a shortcut???
XP enables you to run all windows software, games etc. BuT the security sucks (with Vista, it's intrusive, with XP it's damned hard work to keep the door half closed).
My solution, finally, use Ubuntu for what it does best - internet, some office, some of other stuff. If you have XP installed already, you can add ubuntu as a dualboot option.
XP doesn't need internet - it works best with no internet - no antivirus required with no internet, unless you use USB drives from other computers... but you can plug those in when you run ubuntu - no virus on ubuntu!!!
So, BEST ANSWER:
XP is most compatible, and Ubuntu will give you all the security you need for accessing internet.
step1: install XP, step2: disable internet step3: install ubuntu.
to run both at once, in Ubuntu, install Virtualbox, then install XP AGAIN and run this one ON TOP of your ubuntu desktop.
Right now I am using firefox with ubutnu, and have (windows) Yahoo Messenger sitting on the same desktop!!!
No antivirus, no spyware problems, happy life.
Note: to run XP with 1GB of RAM in a virtualbox, you need 2GB of total RAM. You can actually run XP with 800MB, AND Vista with 1.6GB, but then you need total 3GB RAM.
That's good fun, you can run Ubuntu and XP and Vista.
Personally, I prefer to use Vista to run Yahoo Messenger, it looks best with Vista (on my Ubuntu desktop of course!)
Ubuntu looks nice, it has desktop compositing and management (like Vista, unlike XP) and special effects which run as smoothly as macintosh native effects - unlike added effects for XP (like 'switcher' or 'bricopack').
some screenshots and stuff in this link: