I was first thinking of Gaffer Gamgee and his killing humours, or Forweg who managed to be deadly on himself... but yes, maybe there just isn't that last tint of humour there that would have made them excellent choices over the more deadly ones.
Also, Mount Doom seems to score quite low with me right now. It is decisive, it is great, it is remarkable, great, powerful etc. and the forcing of the ring there brought on a lot of death and the destroying of it as well, but I'd still say it was the ring and it's maker, not the forge itself as such, that was deadly. And like
Lhûna said, you should be courageous enough to pick someone of your size in the first place...
I'm also not so keen on counting the numbers of the deaths but rather the "quality or the execution of them" - if it can be phrased that way.
At the moment I tend to favour Sting, Gandalf the White, Gothmog, Carcaroth - followed by the dragons.
Eowyn was deadly to the Witch King but to none other. Here I think the quality over quantity argument doesn't fit...
Had there been "the oath of Fëanor" or "Húrin's bane" on offer, I would have picked them immediately.
Talking of which, one could say that the company of Gandalf the White is deadly as if you follow him you follow death springing forwards wherever he goes...
Well, I have to sleep on this.