Don't worry about the lame thread title, atleast it covers the topic.
As for best character...hmm, if it must be from ROTK, then I would say Theoden. I often find myself repeating his lines (his pep-talks in Helm's Deep and on Pelennor fields). I love Bernard Hill, I think he is a wonderful actor, I adored him as the crazy scientist Philos in the below average movie "Scorpion King." Also, as mentioned in another thread, I love the look on his face when he sees this soldier fighting ferociously next to him, little does he know that soldier is Eowyn. Theoden's my favorite character, save Boromir
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As for worst, I can't express how much I hate Gothmog. This man that receives one little sentence in the book, gets more screen time then the Witch-king. I hate his lines too "The time of man is over, the time of the orc has come." No it hasn't. Then "Feaaaah, the city is rank with it, eahahaha." His mutilation in the EE even gets me to cringe more.