Windows XP.
I've never used Mac OS, so can't speak for that, but I've installed Ubuntu (couldn't see my drives on my RAID card, even though the Live CD version could), SUSE (can't see the sound card on my tablet PC, nor can I use the tablet pen touch screen features) Windows 2000, ME, 98SE, 98, 95 and 3.11 (and a smidgeon of MSDOS).
XP is the most driver friendly, device friendly, game friendly OS that I've ever used.
Of course I've been using XP for longer than any of the others, so I'm probably a bit biased.
I'm holding off on Vista until the price comes down a bit and Creative release some proper X-Fi drivers.
That said, the thrill I got when I set up Kubuntu on my brother's Shuttle and got wireless working was quite awesome.