Some of my most hated enemies are the undead. Not because their immune to criticals or sneak attack, not because I can't charm them away, no its because whenever I have a character whose sole standout power is destroying undead (lvl 20 ranger with max favored enemy undead and currently a Radiant Servant of Diana (DM's personal Sun God)) the DM never throws any undead our way.
But for monsters I hate for reasons of being evil and such, the last time I had a character death it was to this demon hive queen type monster. I was a sorcerer and melee DPS (don't ask, that party was messed up beyond belief
) So we begin the fight against her warrior type guardians, which wasn't too bad, I was able to surprisingly not die to those, our rogue soaked up most of the damage. Once we slayed all of those though, the queen stepped into the fray, and got a 7 attack FA or something ridiculous like that (we were level 6 at this point). So my character being the smart smart man that he is, throws a LB at her, and then the next round charges up and shocking grasps her in the face. Needless to say the FA dropped me to around -40. We would have had a TPK then if the DM hadn'r had the monster run away, and then it became a recurring villain for the duration of the campaign, which was quite annoying for the players as it wasn't related to the BBEG (Hulks of Zorethra) at all.
The most frustrating encounter I ever had though, apart from those where we had to lose to advance the plot
was in my first 3.5 campaign actually, we were eliminating this apocalyptic cult as they had tried to basically blow up the town we were staying in. So we breeze through their dungeon and smack their traps and minions into submission. When we walk into their HQ type room, we see three people, a cleric in full plate and two fighter type guys. We banter back and forth, I was a Paladin of Aphrodite at this moment, so I did the whole "Surrender and your lives will be spared, but resist, and by the power of Aphrodite's love your immortal souls shall cower before my blade!" Needless to say since they were apocalyptic they weren't impressed one bit. So combat starts, our gnome bard/wizard decides that he wants to play with his brand new staff of fire, so he firewalls the cleric away from the fighters. Good job we all think, lets kill these wimps and then go after the cleric. Suddenly our ranger gets hit for 42 on his flat footed AC. They definitely had an Greater Invisible rogue running around going stabity stab stab. Our party at this point consisted of me, the ranger, the bard, and a fighter/master thrower. So next round comes around and our ranger FA the square he got hit from, and just missed on a chance to crit. Feeling alight with power our bard decides it would be a great idea to throw a fireball right at our ranger to kill the enemy... fortunately our ranger had grabbed evasion by this point. Next round he goes down, I run over and heal, then get done prison style by two fighters and the rogue. Our other fighter had run through the firewall to go dispose of the cleric, and had no idea what kind of trouble we were in.
Needless to say, with no shot to know exactly where this bastard was those of us on the other side of the firewall were terribly terribly murdered by this level 10 (one level below the party) rogue who was using a masterwork dagger. The only reason it wasn't a TPK is the DM gave our fighter's intelligent armor see invisibility at will after he was the only one left alive.