My favorite OS is Debian GNU/Linux; it's stable, you don't run into dependency problems, you can install/uninstall the applications you need with terminals (using apt-get) or Synaptic if you fancy gui scripts. I don't like Red Hat; it's very bloated and feels fake, they also bombard your email with daily updates. OpenBSD is also great; it's the most secure/stable OS (mainly used for servers, but can also be used as desktop).
Of Windows, I prefer Windows 2000 Pro though I hardly use it.