I haven't taken the time since the release of the expansion to look at this, but I always find it somewhat amusing and usually full of surprises. If you look here, you'll find a utility on the Blizzard website that allows you to peruse which monsters in the game are killing the most people.
In WoW Vanilla, the results had settled to a point where you could tell what you were going to find. Like, the Defias typically kill more noobs than Ragnaros ever did, on a day to day basis. Other notables include the Gadgetzan Bruisers, who presumably have a history of ripping people up on PVP servers, for ganking too close to town (or in town).
The biggest killer in original WoW (amongst end game bosses) was undisputably Vaelstrasz the Corrupted in Blackwing Lair, which probably comes as no surprise to seasoned raiders. It was a fight that marked a significant milestone for a guild since… he wasn't simply out to kill your raid, he set out to kill your guild. And that he did, historically, on some occasions. He also had a built in aspect whereas the death of members of the raid was unavoidable. People died like clockwork, and that's just how the fight was.
Judging from the past week, here are some highlights of what bosses (specifically, I'm not interested in how many murlocs killed people, though it's amusing) are killing people these days. This isn't a good 'ranking' so much as a sampling of points of interest.
Maiden of Virtue (334,373 player deaths last week)
She definitely appears to be the Queen Killer at this point in time and an indication that the overall progress of WoW players in End Game seem most likely to be stuck at or around her. This surprised me some, since most of the guilds in Karazhan on my server have beaten her. But then, simply because you've beaten her once doesn't mean you won't still face a wipe next time you meet with her. I wouldn't have pegged her as this devastating, but then here the numbers state otherwise. Her deadliest day was Wednesday (players died on her 72k times that day alone).
Romulo & Julianne (191,287 player deaths last week)
This is remarkably high, considering only a third of the players who do the opera event each week get this encounter. Big Bad Wolf killed a comparatively unimpressive 64,267 players.
The Curator (72, 809 player deaths last week)
Surprisingly low, but I've been able to conclude only one reason for this... The adds are killing people, not the boss himself. I forget what the sparkies are named. I'd suspect The Curator is killing nearly as many people as the Maiden, but I can't verify it at the moment.
Shade of Aran (176,351 player deaths last week, excluding deaths attributable to his adds, which would boost this death toll a little higher)
I suspect people would skip him and shoot for Prince Malchezaar, were it not that the doorman offers a port to his room upon his death. Let me say, I really enjoy this fight… But he's no simple squishy, that's for sure. I anticipate his death tolls to rise (potentially to current Maiden of Virtue levels) in the coming weeks, because he's relatively important.
Talon King Ikiss (176,251 player deaths last week)
I was surprised to see that people are having so much trouble with him (I think he got buffed recently?). He has one of the most remarkably high death tolls of all bosses, currently.
Grandmaster Vorpil (183,332 player deaths)
Blackheart the Inciter (169,063 player deaths)
Murmur (146,737 player deaths)
Well, the Arcatraz is immeasureably harder ~ but… people don't like it. People seem to LOVE to run the Shadow Labyrinth, which has made it appear to be the biggest death trap in Outland! haha