I usually seem to find the best public toilet graffiti at public libraries.... especially big, central, urban ones. Hmmm, maybe I should take some next time I'm there. I wonder if there's a marked difference between the graffiti on the cubicles of the ladies' to the mens'. It would be interesting to compare.
The toilets at my school weren't heavily graffiti'd, and whenever they were, maintenance kept painting over it and ruining our records. This one was pretty monumental - it was like 2' by 3' and was pretty confronting when you were trying to pee. A good amount of squeals came from that cubicle.