I'm currently working through Zangarmarsh on my Hunter, and this weekend I spent a lot of time grinding two completely different kinds of mobs. The Withered Bog Lords in northern Zangarmarsh were cake for my Hunter -- they weren't exactly gentle with my pet, but he was able to eat the damage, and I could have grinded right away on them all day. At the exact opposite end of the grinding scale, however, were those stupid Umbrafen Eels -- not only were they under water (always annoying), but they've got this on-hit electrical damage thing that just drove me nuts. I stayed to play with the Bog Lords for a long time, racking up the XP, but as soon as I was done with the Eels, I got out of the water and stayed out.
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?