I'm going to go with the n-word just for historical reasons. It's rooted in so much that not only does it offend, it brings very vivid visuals to mind.
It's all about context and how the word is used/said. But the c-word is the most offensive to use in public. It doesn't matter what the context is, even muttering it under your breath in public makes it everyone uncomfortable and is completely inappropriate.
Black people use the n-word to refer to each other in literally every way possible. Straight guys use the f-word all the time. People use the m-word to describe things that are simply small. People use the r-word to describe people being idiots.
The c-word is the only one that regardless of context, is completely inappropriate to say, and will make people's skin crawl, men intervene and say "hey, there are ladies present" and mothers cover up the ears of their children instantly.
The r-word referring to mentally challenged people. It alienates them and it is used wrong. Mentally challenged people can't defend themselves, for the most part anyways.
