I think that there isn't any racial, minority, or group slur that can be called most offensive, because any given group is a small part of society. Words that offend them don't necessarily offend people outside that group. The word nigger for example, judging by all the past comments in this thread is the most universally offensive, and I agree that in modern times it's use is almost universally meant to be offensive. But oddly enough, it's not offensive to the group that it's aimed at. It's used used by the very group it's meant to degrade. Kind of like the way we turned around the word yankee in the revolutionary war. Not exactly, but kind of. I was watching Eddie Griffin on Netflix and he was using it as a replacement for just about any word in his vocabulary. Even where most people would use "Um" or "wow" or virtually any noun, he was dropping n-bombs. If it's use is universally offensive, where are all the people coming out attacking him for using it.
To be offensive, I think that it has to hit you directly as offensive to you, not just as an offensive smell that wasn't necessarily aimed at you. I think a the term "Fuck you" is probably the most offensive, because it cuts through all race, gender, age, and other stereotypes to let someone know that you don't have any respect for them and are telling them right to their face in terms that can't be mistaken for anything other than complete scorn. Tell a man "fuck you!" and he will know what you mean and be appropriately offended. Say it to a black, jew, woman, mexican, old lady, child, gay or any other group, they know exactly what it means. Say it to the meanest person you can think of, who wouldn't flinch at using any slur or curse word in the book, and they'll be offended. Call me a nigger and I just become confused, because why would someone call me that. Some women wear the word c*nt as a badge of honor. Say "Fuck you!" to a priest and you are offensive. Even those that would laugh at something like that will laugh because they know it's offensive.