Re: Sexiest/dirtiest song you have ever heard?
"The Difference Between Us" by The Dead Weather. While not being outright filthy, there is no denying the raw sexuality in the lyrics. The driving beat in it punches home the fact that you'd be in for a wild ride.
Though I cannot help but mention the "Mondo Sex Head" album by Rob Zombie. If there was any cd put together solely for the purpose of being played loudly during a lengthy session of BD/SM (or at least very rough) sex, I can't think of any other.