Foul language

Quick question: what's your vote for the most foul word in the English language -- a word so offensive you wouldn't even be inclined to think it, much less say it or write it? Y'all can just put down the first letter and ---.

Funny thing is, I know what word gets TREATED most often as unspeakable ... but I don't think it's any of the words people usually think of in that context.