My KOSNPC would definitely have to be Cenarion Emissary Blackhoof in Orgrimmar. There's a text-wall story that explains why.
When I was yet a lowbie and still fairly young to the game, I was grouped with a big Orc Warrior who was dragging me through Ragefire Chasm to get me gear. Along the way, he walked by the Emissary, roflpwned him in the face, and walked away. I didn't notice at the time that my reputation with the Cenarion Circle had dropped under Neutral. He did this a couple more times in between runs while I was happily bouncing between the bank and AH.
A couple days later, I walked by and didn't understand why the Emissary's name was orange. I went to right-click him so I could talk to him, and I didn't pay attention to the fact that my cursor had become a sword. My low-level clothiness was insta-smashed by the Emissary. I was offended.
When I was between levels 40-50, I went to turn in a few quests from Maradon to some Cenarion Circle reps near the instance. I couldn't turn in the quest, and when I checked what faction they were, I decided I was done doing anything for the Cenarion Circle in an effort to please them. They became my new best enemy.
When I dinged 70, I celebrated by solo-raiding Moonglade. Out of respect for my faction's Druids, I left only the Horde flightmaster and the Druid trainer standing. I dropped my rep with them to KOS, and I've been happy ever since. Whenever I get bored, I take bloody vacations to Moonglade.
I have since found out that I can slowly and painfully raise my rep with them by turning in Un'Goro soil samples to Thunderbluff, but I don't think I care at this point. I'm a Blood Elf Mage. I think it fits the cannon of storyline that I would not get along with the Cenarion Circle, and it's fun to be able to pass on the rep loss when I drag lowbies through RFC.
Note: Do not take free RFC runs from Arcanicus on Norgannon.