[the phantom]

Eowyn- She killed the Witch King... after a Hobbit rendered him killable and paralyzed with a magical anti-Nazgul sword.

Gaffer Gamgee- The obvious humor choice here- his ridiculous sayings would certainly cause one to die a little inside whenever uttered.

Glaurung- Legit deadly. Killed tons of Elves/Men/Dwarves before Angband's gates and at Nargothrond.

Gandalf the White- Certainly deadly, but didn't deal death nearly so much as those that enjoyed it. But I suppose the two issues are separate. He's legit (though I'd rank Glaurung ahead).

Forweg- The leader of a band of outlaws ought to be worthy of consideration, but not next to Glaurung I should think.

Mount Doom- Ooo, interesting... It was used to cover the armies of Mordor in their deadly invasions, and used to forge the deadly Ring, and look at what all the Ring did! Not a conventional choice perhaps, but certainly a fitting one.

Ancalagon- Another dragon! Glaurung was the father of the race, but Ancalagon might've been the deadliest. Of course we cannot know for certain as he was relatively short-lived and his first mission was his last. Picking him would have to be based upon potential and reputation rather than deeds.

Sting- It was put to excellent use in Mirkwood and again at Cirith Ungol, but I don't think killing spiders catapults Sting into the realm of Ancalagon and the like. By that logic we could nominate a fly-swatter.

Carcharoth- He did kill Beren. Except that Beren didn't die permanently. He did kill Huan though, but that point is balanced by Huan killing him in return. Like Ancalagon, it seems he wasn't allowed free realm to use his skills, so it'd be a pick based upon potential rather than deeds once again.

Gothmog (balrog)- Perhaps the deadliest on the list, The Lord of Balrogs- busy killing things from the moment it was possible to do so. Though he had all kinds of help, he still gets the notoriety of striking the death blow on the King of the Noldor twice (Feanor & Fingon), plus the countless Elves and Men (and whatever) he cut down during battles/wars.

So for my money, Gothmog and Glaurung are the top choices, with Mount Doom as a nice outside-the-box choice, and Gaffer as the RL choice I'd make just because it makes me laugh.