Windows, if only for the standardisation, ease of use (it has less annoying non-intuivities than OSX, bizarrely, and shits all over any linux yet used), and breadth of available software. The moral aspect or whatever doesn't enter into it.
As for reliability and all that? XP was going to be, for me, it's last chance to not suck, after 3.x's hail of GPFs and general clunkiness, and 9x needing to be reinstalled every 6-9 months (I think 18 was my record) and festooned with additional memory restorers and the like to prevent everything going to shit. It delivered in spades. If it wasn't for the upcoming barrier of not being able to work with over-2Tb drives regardless of connection (which itself seems to be something the manufacturers are addressing), I couldn't see a reason to upgrade from it - or the 4+ year old laptop I'm currently using upon which it was installed and hasn't yet been refreshed, still running sweet as a honey glazed nut. Which given the sheer number of available upgrade paths is quite some praise. I couldn't even say that in truth about 98SE in its final but far more youthful days, when it was as stable as an inverted pyramid, stuck at half a gig of RAM and a quarter tera (with a trick of my own devising to get it above one-eighth) of disk and very patchy USB mass storage and wifi support with dwindling software and driver coverage.
Linux, been there, tried that, even as a (I suppose now "one-time") ubergeek I couldn't get my nerd sufficiently "on" to really wrap my head around it. It seems needlessly obtuse, and its main tricks - intense reliability and security, a comprehensive command line mode, and good multitasking - have largely been incorporated into other OSes, leaving only the open source bit as its USP, much as you can "sell" an open source product. Which, really, isn't good enough. I'm happy to pay a reasonable amount for a well made proprietary product that won't do my fucking head in when I'm just trying to do some simple things, like, oh, COPY A CD or FIND WHERE THE HELL THAT FILE I DOWNLOADED WENT or FIGURE OUT HOW TO INSTALL AN APP THAT ISNT IN THE REPOSITORY. Plus it happily lets you make (or download) files with names that are completely incompatible with most other OSes when copied onto discs of their native filesystems. Grrraargh!
(This is mostly from experience of Ubuntu, but if the truth is that it's really a fancy skin on top of a particular X-window environment of a common commandline based system, it should hold for all the others. And i've had brief, not really any more positive tastes of Debian, lots of different small-footprint distros (Puppy probably being the best, as you could actually boot from scratch and Get Shit Done without needing a goddamn Masters degree in Ass-Backward Unix Stuff), and really far more than I could ever stand of professionally-applied OpenSolaris *shudder*)
OSX, I got the chance to borrow a Macbook for six weeks when basically needing an extra PC around to do a bulk pile of CD-to-HDD copying. Played around with it, as you do. Happily gave it back with no further desire to Apple-up my life. Pissed me off in a variety of small ways. Maybe if I'd never used any other computer before I'd be more forgiving of its bullshit, but I have and I wasn't. It's only because of exposure to a horrid old Quadra and original iMac at college that I was forewarned of some of the more irritating and retarded concepts in the OS, and was rather shocked that they'd actually been carried over... things not actually closing when you tell them to, etc. It was actually LESS reliable than XP for a few things (e.g. dealing with iffy CDs ... a software-only drive eject is an Extra Bad Thing when the opsys tends to choke on them in ways that only an electronic heimlich will solve) and overall quite obstructive to use for a number of common tasks. No thanks.
Other... well, what exactly would "other" be, here? I've had extensive use of Atari GEM (decent, though sub-win-3.1 in terms of featurage; it's basically DOS Shell for 68k CP/M with knobs on) and a light taste of Amiga Workbench (eh, it's OK), along with more MS DOS than any sane person should rightfully experience in a lifetime... what else would still be in everyday use? Surely no-one uses OS/2 for realworld apps any more, despite it apparently having a committed fanbase and a few niche uses for which its peculiar mix of features are best suited?