Mac OS X, particularly Tiger. That was the ultimate version of Mac OS X, combining enough of the features I use to make it useable, and the unparalleled speed and reliability that I have yet to see and doubt I ever will with another version of Mac OS X.
Tiger compared to Leopard was kind of, as I see it, XP compared to Vista, to a lesser extent. Leopard has all these new features, a better looking UI, etc., but Tiger just seemed to work. No wireless problems, no router incompatibilities, no graphics problems, and just faster speed overall.
I have been using Mac OS X since Jaguar, when I upgraded my fully souped iMac G3 from Mac OS 9.2.2 to it. I waited to upgrade to Mac OS X since there was serious problems with Mac OS X before Jaguar.
I used Jaguar about 50% of the time, with Windows 2000 Professional the other 50%, since Mac OS X still, in my mind, wasn't ready to be used for a full time OS. When Panther came out, I felt by then Mac OS X was very polished, but lacked a lot of features that Windows had, so I upgraded it but still split time with now Windows XP. When Tiger came out, it was basically heaven falling from the skies. It was even more polished and reliable than Panther, but brought a lot more features than expected. I didn't upgrade it until I bought my 1st generation BlackBook, because by then the G3 era was dead, and I couldn't have been happier. Tiger has made me use Mac OS X 100% of the time, except for the occasional Windows game.
When I upgraded to Leopard, I haven't had the reliability of Tiger. Spotlight is much faster, and Mac OS X UI looks better, but besides that I don't use many of the features because I find them unnecessary. I immediately had Wireless unreliability, and still have it to some extent. Occasionally the OS will lock up for a few seconds, and RAM usage uses about 150% of the RAM Tiger uses.