the best overall is Mac OS X, with Mac OS 9 coming in at second, as far as *overall quality* is concerned (ease of use, app support, stability)
Now, if you wanna know the just absolute best... OS X is still top dawg, with any of the BSDs and Linux tied for second. Darwin, the BSD UNIX base of OS X is lumped in with BSD and Linux, because without Quartz, Aqua, and all the app frameworks... it's not OS X