There is good and not so good in all of them, although I think Apple's OS X Leopard is still the most elegant mainstream operating system and Windows Vista is nowhere near as bad as critics say.
Microsoft has introduced some significant improvements in Vista, including an updated user interface and better security, although some important planned changes were left out. Vista also requires considerably more PC resources than XP to run smoothly and did not always interact well with third-party equipment drivers. Hence much of the complaints that have dogged Vista and encouraged many existing PC users to stick with XP.
Now it seems Microsoft is in a hurry to roll out its replacement, dubbed Windows 7, which arguably would be more appropriately named Windows Vista Service Pack 2.