Imo Blizzard has really stepped up the effort they've put into voice acting in 3.3 in comparison to the last few patches.

My favourite's probably Blood Prince Valanar in ICC. I have utterly no idea what "My cup runneth over" means, but the line's delivered with just the right amount of sneering elvish arrogance and subtle bloodthirst to add a lot of atmosphere. Whenever I hear it, I immediately get a mental image of a cup of blood spilling on to the floor. "BOW DOWN BEFORE THE SAN'LAYN" also scares the %#!! out of me. Reminds me of Teron Gorefiend :P

NOOOO! You kill Stinky! You pay!