In a recent question, I realized that while I know what's currently considered the most obscene word in American English ("cunt"), I am told that word is much more unexceptional and workaday in British English, and I do not actually know what word does bear the distinction of most obscene in that vernacular. So, what is it?
Looking for current rather than historical usage, obscenity rather than taboo for reasons other than obscenity (as with racial slurs in the United States), and obscene words rather than obscene concepts -- that is, the taboo attaches to the word itself. If it varies between major British English dialects, give me whatever flavors you know.
For purposes of this question, "most obscene" means that somebody using the word in a context where obscenity is not normally expected will most commonly be thought to be performing the greatest violation of social convention for reason of the word's obscenity.