Literally dirtiest is probably the time that a banshee turned the house I was staying at into a swamp. But metaphorically dirtiest...mmm, the 1960s. I was head of a band at that time and we'd just come back from tour. It was me, my bandmates, some groupies, and a few local girls one of the guys knew. We were holed up in this absolute shithole of an apartment but, most importantly, we were holed up in this shithole of an apartment with my drug dealer. We spent the next few days indulging in every terrible stereotype of musicians, especially those in the 1960s. I don't exactly remember what I took or who I fucked, but I can safely say it was divine.

Runner up is probably 2007, in a club bathroom, with both of us off our tits on cocaine.