I don't know about 8 yet. I have heard a few bad things about it but I'll reserve judgement until I see it.
I had XP until very recently (May this year) and it was always a good reliable OS. I have Vista now (because I already had a disc for it and didn't want to buy 7) and I actually quite like it. Any problems it has I can easily deal with, and it's behaved pretty well anyway- barely any more trouble than XP. I also like the Vista/XP style taskbar better than 7. However, 7 is much more reliable and user friendly than Vista, so I'll go with that.