if you're lookng for games, Win98 SE.
if you want apps, Win2K PRO.
if you don't have any money for either, Linux(don't get me wrong, i love linux, especially because it's free, unlike the next choice)
if you're running a server and have an unlimited budget, Win2K Adv Server. with a budget: linux baby, yeah=)
this is just imho, and i keep in mind that i haven't used beos or DOS enough to know anything other than how to get around in it.