I'd have to say Prince Imrahil and his knights of Dol Amroth. Not so much Imrahil, because he gets a lot of acknowledgement in ROTK. But his knights (beside Gandalf) were the only Gondorian soldiers able to withstand the nazgul shriek. Imrahil I also love about him that he doesn't impede Aragorn from claiming the throne, he actually urges Aragorn to do it
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Ghan-Buri-Ghan, need I say more, I just love the way he talks, "gorgun."
Some other characters that I think are overlooked are (that aren't already mentioned):
Butterbur
Farmer Maggot
Beregond
Radagast
Hama
Erkenbrand
Ohtar
I can go on for a while, but anyway...
I can see that we have a Celeborn fan wagon gathering up, I might as well hop on, GO Celeborn. Snowdog, wonderful answer
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