Like cait-sith said, that's sort of like asking "Is xxx the greatest vehicle ever?" There is no "correct" answer to that question, because (like with family cars, sports cars, trucks, jets, and space shuttles) there are so many different purposes for an OS that there isn't, and I don't think ever will be almost by nature, a "best" one for every purpose.
OSX does combine a lot of good stuff--it offers a nice interface and ease of use for the very inexperienced, it offers a reasonable amount of GUI and under-the-hood power for creative professionals, and it offers a pretty solid level of geeky goodness for the geek. And in the "production" arena it has the underlying server architecture (in Server flavor, of course) to handle most things there, with the added bonus of some fairly easy to learn admin tools. Not as much handholding (or bells and whistles) as Win 2003 server offers, but much simpler.
Anyway, I would say that it is almost certainly the "greatest" consumer OS extant, but I won't go any farther than that. It's got a good balance, but in most areas there's probably another OS that can best it, at least in theory if not in practice (for example, the BeOS has some sweet data management features for the professional that OSX doesn't offer, but in practice there are no major apps available for it).
Also, I expect OSX, or at least its core, to be around for quite some time--after all, it's essentially a souped-up Unix variant, and various flavors of Unix have been around since the 70s. It's a mature, tested foundation on which to build. Still getting the job done, albeit in a barely recognizeable form for someone who'd used a Unix in 1980.
