The worst for me had to be one of those 'desperation' pizzas, either the 99 cent "party pizza" from the grocery store, or a good ol' heatlamp slice from some place I've never heard of in an airport somewhere.
I noticed recently Subway has pizza near me now, as much as I am not inclined to try it, I gotta believe it would make the list.
The other one that comes to mind was when I lived in Phoenix, we ordered canadian bacon, but got regular bacon instead, lots of it, very greasy, and they burned the pizza besides. We were on the way to a ballgame and so that was dinner....