Many choices but I could try to pick some that are the worse which are:
Shaq as Henry Irons (Steel) A terrible costume, stale acting, a bit childish and the dumb basketball idea. We know he plays the game why put it in every film of his, even his commercials are terrible.
Arnold as freeze (Batman and Robin) terrible suit, corny puns which freeze has no emotion so not sure why he would be making them anyway. He is a troubled soul, not a comedian.
Justin as Son Goku Dragonball Evolution) looks and acts nothing like Goku, more horny than powerful. Bit of a jerk in film too. Cheesy dialogue or overacting. Not even close to anime material.
T. Lee Jones as Harvey (Batman and Robin) terrible makeup, cheesy jokes, not terrifying at all or threatening that doesn't do much except steal which does nothing for film.
Eisenberg as Lex (Batman v Superman) way over the top but kinda like it some performance, prefer original Luthor in movies. Way too hyper and way to easy Lex did everything in film with no explanation. A bit creepy at times too and not in good way.
All cast in reboot fantastic four (Trank version) Cheesy acting, boring, look nothing like the characters, terrible costumes, hate the plot change and always inside the damn building. This I vote as #1 worse movie ever, even Dragonball Evolution was better than that and I at least enjoyed Catwoman more. I watched this twice a i fell asleep twice halfway into movie and still never cared to see all the way. Thing had no pants and the science fair was stupid, kid actors are totally not believable. Doom suit look like someone burnt in a microwave accident.