Well, its hard to pick one. Do I go with a movie I knew was terrible, but watched out of morbid curiosity? If so, then the religious propaganda piece God's Not Dead would be the worst film I've seen, mostly for being a really nasty pile of shit (that would be laughable if I didn't know everyone involved believed the shit they were spouting, which is what really brings this movie down from terrible to the worst). There is also Catwoman, which I also went into knowing it was crap (but its still better then GnD).
There are also some real awful comedies, like Meet the Spartans, and I was young enough when I first saw it that I thought it might be good (teenage me thought that stuff like Family Guy Scary Movie was funny, I had shit taste), but in retrospect its one of those "2 of the 6 producers of scary movie" movies and they're all obvious shit to anyone who isn't a doofus, and it was the movie that made me realize that I couldn't stand that style of humor anymore (I was 18 when it came out).
In the end, when it comes to movies I didn't know would be bad before seeing them, there is an "unholy trinity" to go for the worst:
Goodburger, which I didn't choose to watch,knew nothing about before seeing it, and absolutely hated it.
Transformers: Revenge of the Fallen, because the first Transformers movie wasn't that bad so when the sequel came out no one was really aware of how bad it would get.
Southland Tales is the most incoherent pile of shit I've ever seen. Not even the Rock, Buffy and the Highlander could save this pretentious, nonsense garbage.
So, basically, the worst movie I've ever seen is God's Not Dead, but the worst movie I saw without knowing it was going to be awful is a tie between those three.