There are so many choices here but nobody thinks of them because they are so bad they are out of collective conscience. Plus, most people haven't played them.
I will go with Cactus Jacks. What a turd.

Shaq is bad but I think CJ is worse. There is no way anybody could convince me that CJ is even half a turd. There is an arcade by my parent's house and CJ is usually the only pin which works. Probably because nobody ever plays it. So every time I go to that arcade I put a few quarters in just to see if I will change my mind. But every time I finish thinking this is the worse game I ever played. There are a lot of so called crappy games I have never played so I won't talk about those. But CJ is the bottom of the barrel for me.
TMNT and BBBB are barrel scrapers too.
Edit: Come to think of it, BBBB would have to be the worst game I have ever played. It is sooooo boring and the layout is just crap. And not little bunny crap either. Yup, gonna have to put this ahead of CJ as the biggest turd.