TWRoO on 31 December 2009
Depends what you want it for... if for something specialist and you know what you are doing, then some version of Linux.
Windows 7 is a good and easy to use all rounder. XP and Vista are still perfectly functional (though if you do have a relatively modern computer I would reccomend the upgrade to W7)
Mac OSX.... well... I wouldn't bother.
----
@Severance.... What are you doing with your PCs that means Windows crashes every day? I think I could count the number of crashes I have had on my fingers from XP... and none so far from 3 months of W7 (not to say it won't in future)
We did have Windows Me on our old PC though, that was a bit of a pain.
Essay Pro