most annoying in nm: frostmaw (dp parade) and shards (dp parade PLUS walking miles and miles to get to the guy. at least he does not regen his life). probably due to going in there with bad builds not knowing what to expect -_-
HM:
vloxen (WOW slavers groups outside slavers exile---what a cute surprise! minions everywhere especially that one necro boss that clumps up near the res shrine with his 20 minions---make sure to move the mob away from rez shrine if you're gonna die)
rragar's. the 2 necro bosses in proximity to each other, enough to not choking gas them

need i say more?...ok i will

those ray of judge spiking hounds. rragar himself and his buddy were so easy...but everything else? glad i had candies to eat down there. i dont go anywhere without my candy in hm. (hench bombs are fun to watch, i guess...)
edit--
added shards of orr (removed ooze pit). even with 3 smiter heroes it was a nightmare ._. the groups picked off fast (smite spikes), but there were so many huge groups--they ate thru my bunched up ball of h/h. the boss was not nearly as hard as i expected, but i cut down the time to kill him by 3x that of normal mode

hooray for candy and powerstones.