"Fuck". It is beautifully ubiquitous. My runner up is most likely "goddammit", although I'm not sure why, since it's a relatively minor swear, compared to my #1. From there, it's shit, dick, and all the rest of the swears are in some 5% at the low end. Surprisngly, I never seem to use "****".
Yes, I keep track of this sort of thing. It's important.