I love hearing Time Warp kick in.
The harmonious sound of a druid's tranquility being channeled :-P
You face jaraxxus!
The old sound of Cloak of Shadows (and I believe Shadowform as well). Bummer they changed it in Cata.
The levelup ding and achievement swoosh.
Baby murlocs, the sound of hooves on metal, ALL of the music played in Tanaris.
Oh, and Lorewalker Cho, but that's mostly due to the MASSIVE nostalgia aspect.
Gnome death sound !
My favourite sound still is the one from W3 ROC intro ( one of many dungeons )
If you know what i mean
I've always loved the sound your bow makes when you prepare to shoot with it. Been in love with that sound since the first time I played the game!
The sound when you are sent back to the login screen when you d/c in the middle of a raid and cause it to wipe and get kicked.
I always like the sound of tauren males dying with the THUMP, I use to love always killing them when I use to be Alliance just to hear the sounds. Now that I'm Horde, I don't hear it as often D:
As a side note, can you imagine the sheer amount of time it has taken to create ALL of the sounds in WoW? There are thousands of unique sounds for spells, npc's, events, and all interactions with the world. It's pretty amazing really, if you think about it.
also...."SOCKS!!! NOOOOOOOOOOOO!". too funny
The Murloc babies and when I pop bloodlust on my shaman