The ones that have stuck out were:
Linux. Ive used Ubuntu for 5 years now and its pretty much perfect
Amiga OS. Simple and lean.
OSX seems nice, and I enjoy using it on my wife's laptop, but I've never really spent a lot of time on it, other than basic tasks. Doesn't seem to offer anything that makes the loss of freedom worth the trade.
Of the windows versions, 2000 is the only one I'd consider using if I ultimately had to.
