uh, we all know that vista was a flop, but xp was possibly the greatest os ever designed - longevity, scalability, etc. beyond that, you've got ME (which was really just a service pack) which sucked, then 2000 which was pretty good, then 98 and 95, both of which were pretty good. 98 was awesome, actually, i still run it on two machines. oh, and don't forget server edition 2008 and 2003, both of which are excellent.
again, read above. they 'screwed up' one major release and one minor release in ten releases (if you count service packs as releases, which ME basically was for 2000). i'd call that pretty good.
argh, i'm defending microsoft now. shoot me.