If you don't play games on PC i recommend you go with Ubuntu since it's free, has great support and it's very easy to understand. You can always setup a virtual pc and run WinXP, Vista in there. No more problems with spyware, viruses etc
I'd have to say XP, stable and well supported, not ready for the Vista switch yet. As for Linux i don't think its ready for widespread adoption as a desktop OS, server wise its great.
I'm still using XP because of all the complaints about Vista. Plus I can get more out of less hardware.
It's really all about what you want your computer to do - always has been, though nowadays computers are so fast you can do just about anything within reason, and that reason will always apply, however it would be quite interesting if Operating Systems were once developed in such a way that nothing is limited due to support. If for instance a simple Operating System like CP/M was built to handle 2Gb of Memory a Terra Byte Hard Disk, 50Zillion Hertz CPU and applications were written in the traditional language made for Computers (assembly) it would be interesting to see how powerful an application would be compared to it's Windows counterpart! :-D Having said all that it depends on what the end user wants, I use Windows XP since it came standard with the computer and seems to run efficiently on it until some additional 3rd party Hardware Drivers etc have played gremlins with the system! :-o I could probably get away with some simple DOS since DOS does give support to Internet, I could write Applications, do some art, though making movies and doing stuff with my Digital Camera would probably be out. I still use DOS for games though - Windows 95 seems to fall flat on games! CP/M can do stuff similar to DOS and has the ability of being quite capable of doing what DOS can, though programming those tools is another matter. CP/M has a wide range of Programming languages available for it ranging from BASIC, C, Pascal, FORTRAN, PL/M & PL/I & Assembly of course just to name a few. Programming is a hobby I do very occassionally on the side when the environment gets chaotic. For Applications CP/M has a good range of applications with the exception of the Internet, there's a wide range of tools available. In terms of GUI appeal, CP/M is text orientated, the closest thing for GUI support though is under DOS Plus 1.2 or CP/M-86 v4.1, which can run GEM. Under GEM again a wide range of Applications are available, some support for the Internet is available I believe. DOS Plus 1.2 is derived from CP/M-86 and supports the earlier CP/M-86 ".CMD" executable files as well as some DOS ".COM" files (which are derived from the 8bit CP/M ".COM" file and limited to 64k), don't believe this version of CP/M-86 v4.x supported ".EXE" files. When GEM comes into play, again all sorts of things are possible with it, like earlier forms of Windows (vers 1.xx, 2.xx & 3.xx most notably), GEM isn't a true Operating System - rather it's an Operating Environment, however it's not limited in the range of applications possible for it, it's also possible to write new applications for it - my favourite being Turbo Pascal 5.5 with an Unit file to generate GEM files! :-D