XP isn't the worst OS I've used. Win2K is my favorite, followed by OS 3.0 (relative to when it was released, of course). I'm also quite partial to OS/2, that is, V2.0, which is driven mostly with text-rendered user interfaces. I love a good TUI. I haven't tried OS/2 Warp, but I would like to, just for inspiration. :-)
Win98 was too flakey to be "better" than Win2K. Just last week, I emptied the Recycle Bin on my legacy computer, and now I have hundreds of cross-linked files on my hard drive and all sorts of other filesystem nastiness. That box also crashed on me about 4-5 times a week. I don't think I'll bother re-installing it. I only really play DOS games on it, anyway. Anything before Win2K is junk. ;-)
Any Unix-ish OS is obsolete for my tastes. Typing is what you do when writing scripts or your video drivers are corrupt. Otherwise, give me Directory Commander, any day! Gnome also drives me nuts. What ever happened to TUI's?
I also liked DOS, believe it or not. It might be braindead, but It's so simple, there's really nothing you can do to permanently mess up the system. DOS was a good gaming platform, at least.
The worst OS I'm familiar with has to be NT4. It's not as stable as Windows fans want you to believe, it allows drivers and apps to do all sorts of stupid stuff to the system, corrupted installations with NTFS have to be wiped out, as there's no way to access a partition from DOS without 3rd party tools, and driver organization is horrible. I'll never understand why 85% of drivers can be installed as "devices" and "services", but specialized drivers, like video and hard drive controllers, have their own control panels. IDE drives are also labeled as SCSI devices. In fact, even parallel port card readers are labeled as SCSI devices! :-?
Second worst is just about any old version of MacOS. Slowest OS's in the world, and loaded with bugs. I ran into a problem when I installed MacOS 8.0 from scratch on nine identical Power Macs. On *FOUR* machines, the CD-ROM never showed up unless you boot from the drive, and trying to play an audio CD locked up the machine. The other five Macs worked fine (relatively speaking). Every Mac person I talked to told me it was a hardware problem.
Installing the OS 8.1 patch fixed the CD-ROMs. Still, our Quark Express CD's didn't show up when you put them in the drive, while all other CD's worked fine. I had to use some trickery with unfolded paper clips to install Quark. Ugh. And of course, they crashed, like, ten times a day. It burns me up when fanatics tell me their Macs never crash, and they are soooo easy-to-use. Mixing and matching hundreds of system extensions, trying to find the "bad" one, is not what I call easy-to-use! But hey, who cares now that OS X is available?
That's not to say I just hate Macs. I really hate just about everything. ;-)