Prince Imrahil of Dol Amroth.
He's pretty much a perfect knight. Shining armour and all, leading charges, rescuing dragons and slaying maidens. Or at least saving Eomer and his Rohirrim. And Faramir earlier on. And he noticed Eowyn was still barely alive and needed healing so I guess she sort of counts too. That's 3 main-ish character deaths prevented by one very mortal and otherwise unremarkable guy. Generally a pretty decent list of accomplishments for a lifespan as short as a humans.
Runner up for my nomination would be Feanor, Elf berserker kills a host of Balrogs solo including Gothmog, their captain. I also like him because of his many, many faults.