for me that would have to be the original windoze 98, ran it on my older box which now has 98 SE and redhat 7.2 on it, usually would get a minimum of 3 or 4 BSOD's a day and several of those little white ignore / close boxes too

second one (only just though) would have to be XP, but only because of all the trouble with drivers and software not working properly and its imcompatibility, apart from that setback i'd almost say M$ did a good job