Fighting these two back to back got me thinking: what's the easiest/hardest mob to grind on? Some mobs (like the tigers in Stranglethorn or the bears/spiders in Ashenvale) are super easy to grind -- they have no special abilities, they're spaced out, and they go down fast. But others -- most gnolls, in my experience, and lots of casters -- are just annoying as all get out; they run around, pull others, heal up when they're almost dead, and give you lots of headaches while taking them down. Those are the mobs you don't sit around and grind on -- they're the ones you avoid completely after you've done whatever quest requires you to kill them.
What are your favorite or least favorite mobs to just grind on in the game?