Right now? Mummies. My 5th level party just fought one and, while they won... I don't think it was particularly fun. Particularly for the rogue - its AC was too high for her to hit consistently, its DR and undeadness made it almost impossible for her to damage, it could hit her fairly easily, and she was unlikely to make Fort and Will saves. The only useful contributions she could make to the fight were things like flanking the mummy to give the other folks +2 to hit, and while that's useful, it isn't fun. And other than the sorcerer, nobody could do much against the mummy... good AC and heavy DR, nobody wants to melee with it, and mummy rot is very, very nasty to try and adventure with.
Pretty much anything with save or die/save or suck abilities would be on my list. I try to avoid using those, especially save or die, for two reasons:
1) Save or die just seems to be a 'DM vs. Players' sort of thing. If you die because you've been hit a bunch of times, that's one thing, but dying because you failed one fort save that you couldn't avoid just feels different. And if its a targeted ability, then it can seem that the DM was gunning for a player.
2) To spare myself from having to find a way to work a new character in. If we were just doing a huge dungeon crawl where we were mostly ignoring plot and character backgrounds, this would be different, but if someone gets hit with a save or die in the middle of a dungeon, in a time sensitive scenario, and/or if they're a character who is becoming important to the plot, it can be tough to work in a replacement character.