I'm sure what I've seen so far as terrible restrooms pales in comparison to what guys have experienced with what I've heard of men's restrooms, but I've seen a few types that stick in my memory.
Little Girl Assaulted - These aren't that bad. Maybe they have some water mess but you can tell unsupervised little girls have played games with the fixtures, the stalls, and the paper. There are especially high school girls restrooms which have this too...only toss in more...nastier remnants carelessly left behind.
The Standard Gas Restroom - Cooooolllllddddd...for one...when they have seats. Many of them are unisex, so I learn to expect anything. The best of these just have toilet paper that is like cotton fluff pressed together. I could sneeze and it would fly apart. The worst of these make me wonder if I'd be better off chooping in the bushes behind the station (and why I studied a book with urinary muscle training and other methods for situations where a clean area to sit isn't available)
Bad Station Restrooms/The County Service Rest Stop - You know how people wonder "does a bear shit in the woods?" Well, it doesn't, IT GOES IN THE COUNTY REST STOP X_X!! (specifically in Missouri, Oklahoma, and Indiana). A communal, stone block lined with lime would be nicer. So would be water...doors....anything...
The Worst - *shudders* Here is the reason I wrote this. I actually recently managed to remember this one. I'm sorry I did.
I could taste it in my lungs. It cannot be that such a foul place could actually exist. You know the rotten landscapes of Silent Hill? It must've been inspired by this. Standing in this place for more than a few seconds should be classified as self-torture. Rats and roaches and other beasties surely have sworn it off out of fear.
There are aspects of this bathroom, a bit outside of Amarillo, Texas (local gas station brand place), that I have blotted out from my memory. There are things that I saw in this bathroom that I cannot describe. I'm glad I was only in it for about five to six seconds. It was more than enough for a lifetime. Even with my micro-bladder, I could hold it to somewhere else.