For the level, the stats say that Defias Pillagers are among the hardest pull in the game. That's probably because they're the first casters players face, not to mention lots of players head towards Deadmines too early, because they're anxious to see the game's instances. According to the "casters are hard" rule, I'll also throw in Murloc casters-- they come in packs, they run away (pulling more), and they hit hard (that gurgle is one of the most hated sounds in the game).
At the higher levels, the Lava packs in MC have gained a reputation for being hard pulls (mostly because of those Flamewalker fireballs). As for the other endgame instances, I can't think of a harder pull of non-boss mobs-- my guild handled the other stuff pretty well (err-- when we didn't make mistakes, that is). The ZG bats are annoying but definitely handle-able, and the AQ bugs are tough to pull one group at a time, but once you've got the right group of them, they're easy to whittle down.
But for my money, the hardest non-boss pull in the game (if you're not ready for it) is the pull in UBRS we usually do right before the Beast. It's the group of three dragons and two orcs on the far side of the room across from the Beast's lair. If you're not well-geared and/or ready for it, I think that single pull is a real test as to how your group can handle focusing fire, crowd control, and aggro and healing management. So what do you say?