onyxia wins by far if you count total encounters and not just official lore examples.
onyxia level 60, we murder her and hang her head up in stormwind
onyxia level 80, we murder her and hang her head up in stormwind
onyxia in the comic books, varian murders her and..you know
onyxia level 85, deathwing sews her head back on. we murder her and hang her head back up in stormwind.
zalazanes is my favorite though. noobs killing him and bringing his head to the questgiver....dozens of times. each time people coming back with a bewitched coconut or some troll minion of his' head.