There's a fun little discussion on the forums right now about the best noise out of a mob in the game, and people have all kinds of different opinions. Among the many brilliant things Blizzard has done with World of Warcraft is the way their sound design makes each and every encounter with a mob very memorable, so that even if you're not paying close attention, you know a) when you've aggroed something, and b) what you've aggroed. We all know what a wolf growl sounds like, we know what it sounds like when the caster you're trying to sneak past starts casting (and many of us can probably tell what spell, just from the sound), and of course, probably most memorably, we all know what that gurgly murloc sound means, especially when you hear it in numbers.
Lots of the best audio clips are actually from bosses (all of Molten Core's bosses were memorable, for some reason, and "Too soon!" has even reappeared in the game since). My personal favorite lately is the quote from Elder Nadox, the first boss in Ahn'Katet's Old Kingdom. He shouts an otherworldly phrase (apparently in Nerubian) and it ends with a "k-k-k-k-k" sound that must have been edited together -- it definitely doesn't sound human. Every time I hear that (and he says it a few times throughout the fight), I get the feeling that we are fighting something that is very, very old and very, very evil.
But of course lots of the audio cues help pull you into the game like that. What are some of your favorite mob noises in the game?