I know we all hate the ghosts and wraiths and generally all the incorporeal undeads that don't take any damage from your beloved sword, and drain your ability stats and heal themselves up afterwards, so that little damage you managed to deal them means nothing.
But they are nothing compared to their evil elder brother; the corporeal undead. My characters usually do pretty well until they meet the feared corporeal undead. I now have 5 death/near death experiences with undeads, but none with other creature types.
It may be that undeads are always filled with diseases and other kinds of fortitude saves. i always blow my fortitude saves. I could be Lord Constitution, the Superstitious Barbarian of The Great Fortitude and still blow every single fortitude save. They also have a habit of lying perfectly still waiting for adventurers to go near them, and then make a full attack and outright kill the poor human. Sometimes they disguise themselves like humans with only a little pale skin. The guy you thought was a wizard, was suddenly ghoul dropping its quarterstaff and mutilating the poor human(this happened to my character last night). Now I think I will always carry a scroll of detect undead, so they can't do this to me anymore.
How about you? What are your most hated enemy and what do you do to counter them(if you do anything).