My Picks (from what I have used):
Mac OS X 10.1 had the cleanest implementation of the Aqua UI, there were no metal windows, unified toolbars, non-standard toolbar buttons in Mail, purple scroll bars, black sidebars and a whole lot of other inconsistencies Apple has introduced. It was also the first stable version of OS X, the first to run Microsoft Office and Photoshop. It wasn't done, but it was the first OS X version to be good enough to use day-to-day and that was significant. And the contrast with what we had been using was stark. Aqua looked lovely.