I'm currently using Windows XP, Mac OS X and Ubuntu 11.04
I think it's fair to say that they all have their pros and cons. I use XP as the main OS on my laptop (which happens to be a MacBook Pro) just because I always have and probably always will love XP. It lets me play all my old PC games with no problems at all.
OS X is good because if I'm running off of battery power the machine will stay alive for a hell of a lot longer. Plus, when it comes to using the terminal, OS X is far nicer than the Windows command prompt in my opinion.
Finally, Ubuntu is just awesome for me. It has a lot of features that make life easier when it comes to getting my uni work done. I can set up folder shortcuts in my file space that will automatically connect to the university servers and show me all my documents and let me work on them for example.
There is no "best OS". It depends entirely on what you want to do with it.