Leopard has it easy, it's all running under a controlled environment. XP has to be adaptable, and it is fantastically so.
I wouldn't say Xp did anything fantastically to begin with. To begin with it was internet naïve, unprotected and had a high number of vulnerabilities and exploits. By SP2 most of this was under control. The much maligned Vista is far safer and secure out the box than XP. Likewise, aside from a funky colour scheme (Microsoft's attempt to mimic Aqua without the sophisticated compositing, translucency and shadows) Windows XP was very similar to what is regarded as the best version of Windows ever: Windows 2000 (NT 5).
As for Leopard having it easy: this is purely an engineering decision now. Despite this I'd also note that many people seem to get OS X on hackintoshes and netbooks with little more strife than it took to get the average Linux distro running a few years back.
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.
Mac OS 9 is a good shout as well, because configured correctly it was at least as stable as the DOS based versions of Windows it was competing with, yet it added new impressive features like USB printer sharing, multiple users and the keychain to what was essentially old, fairly neglected technology. It also allowed for the transition to OS X by running carbon apps and running inside the classic emulator.
