the worst? well...
back in college, there was a group who played AD&D. these guys made the knights of the dinner table (and the black hand) look good by comparison. One of them always played a drow assasin. another, had a CN mage who always wandered off alone, the rest of the party unconsious bleeding and polymorphed, to fight what just kicked our collective butts.
one game of rolemaster, it went really bad. first, the drow (CG alignment) kills my character with a level 50 poison. his only explanation? i wasnt 'doing enough'. so then we go on with the quest, and meet a dragon- who will let us take all the treasure, minus a couple items. one guy has his character (some kind of dancer), attack the dragon. big fight- the drow sits there and defends, my char gets thretened at swordpoint by another party member, and the fight goes poorly.
then the dragon gives us a chance to surrender. the wolverine-lemming, of course, refuses, so we get hit with a powerfulll spell and all die except the drow- and my new character who had enough healing magic to survive. we lose all our gear, and the campaign ends mid-qeust. and the guy who started it? left mid fight and didnt come back.
same group, the drow would often have his character commit suicide around 1 or 2am, when he got bored. the gm- well, he was just as bad. i had a lightning bolt BOUNCE off of a t-rex in AD&D. i eventualy failed my saving throw- he made me roll about 20 of them, one for each time it bounced between me and the dinosaur.
the last game i played with that group, and the reason i quit playing D&D for several years- a group of 10 players, 1 evil, i was good, and the rest were chaotic neutral. went downhill from there.
whats more- the drow player? out of character, he made death threats to me, that he would one day be a hit man, and kill me first. yes, really. not a nice guy.