Not willing to go into the discussion as whether Ancalagon's potential should be more deadly than what Glaurung actually did, or whether Gothmog would outweigh them in deadliness - and where would Carcaroth then fall on this line - I need to set all these great candidates aside.
Had someone suggested separately whether any one of them would be a good card to play for "deadly" I would have agreed enthusiastically that you had a fitting card indeed - but too many of them just messes one's head.
Even if I kind of liked
Lottie's idea about Eowyn's innate deadliness, I must discard Eowyn as well. It's true The Witch King could be killed by "no man", but then conversely it means he could have been killed by "any woman"...
Leaving Mount Doom, Gaffer and Forweg out from consideration on grounds I made already before I'm left with Sting and Gandalf the White then.
Sting is a deadly weapon indeed, a cave troll and Shelob learned that. But it just doesn't feel like being enough.
Gandalf the White, then again, draw death around him anywhere he went being responsible for thousands of deaths all over the place: where he rode, death and distruction followed. Also as Gandalf the White he sure was a deadly combatant also in himself: he killed a balrog as a grey version, so what could he have done as White one? It's hard to see many who could have gotten alive from his hands if there was a fight.
So without further ado... (Needs to go to choir rehersals)
++ Gandalf the White