I think it's funny that you asked about the most inapropriate rock song and some answered rap. Whoopsies.
I think "The Ballad Of Jerry Curlan" by the Angry Samoans fits the bill. I won't even attempt to put some of the lines becuse it would be entirely asterisks with the occasional word that doesn't get ommited, like "suck" and "sister" and... well that's about it from what I guess would be the chorus. The bridge is done like a melodramatic ballad about a really nice guy Jerry Curlan who everybody likes, then on the chorus they start shouting all sorts of downright perverted things he does in an angry mannor. Quite funny though.
The second place would again be rewarded to the Angry Samoans for the song "Homo-Sexual" which is a little less vulgar (a little) and very very funny. It still makes me laugh out loud everytime I hear. I'm probablt building a bad name for the Samoans but it's not like there were people lined up to get their albums anyways. But there should be, a great garage/punk band with great funny and more serious songs.
Edit: I love that Wayne County song by the way, it's made all the funnier by the fact that he/she is a transexual.
And "Heroin" was done with the Velvet Underground, not Lou Reed's solo career, just to make that clear. And I never thought it was too inappropriate, it's a great song though, I recomend everyone to go listen to it, no joke.
And I'd like to add that it's great to have rude/vulgar songs adding color to your library. And if it's just a joke you shouldn't get your undies in a wad about it, it's good for scaring the conservatives and adds some humour to music and life. I don't know what I'd do without Iggy Pop, the Circle Jerks, and the Angry Samoans trying to be controversial, the world would be a less exciting place.