I'm not sure which is most frequent, so I'll just have to list some that see somewhat frequent usage. I don't actually swear that much, but I'd say that I curse for several different reasons, and perhaps that's the cause for the large variety on the list.
Fuck
Shit
Shite (Sometimes pronounced "Shy" as I'm a bit lazy about pronouncing my t's with my Midwest accent. Though it may also be that I've had cause to curse my shyness now and then I'm sure many Escapists can relate.)
Son of a ***** (Sometimes contracted to "Son of a...")
Dammit (Sometimes "God dammit" or "God fucking dammit")
Christ (Occasionally "Bleeding, bloody Christ")