Poor, poor Mankrik.
My choice would have to be Rhonin, leader of the Kirin Tor. That guy was way too much of a badass to be killed off in the battle for Dalaran. All the work he did to save Azeroth in the "Well of Eternity" series... it's a shame to see all that greatness just die off in some side story that only lasts like 5 quests. Lame-o.