Batman & Robin tends to top everyone's "worst" list, but there are many other candidates...
Everyone loves the first two Superman films starring Christopher Reeve, but the wheels started coming off that franchise by 1983's "Superman III" (co starring Richard Pryor). Even worse was "Superman IV: The Quest For Peace" (1987) which simply looks cheap and barely makes any sense besides. Even Christopher Reeve looked like he was just in "IV" for the paycheck.
1991's "Captain America" starring Matt Salinger (son of author J.D. Salinger of "Catcher in the Rye" fame - there's some trivia for ya) was a low budget stinker of the highest order.
Ang Lee's "Hulk" (2003) was a mistake. I wonder whose idea it was to give a $100 million summer blockbuster to a Hong Kong art-film director who'd apparently never read a comic book in his life.
1993's "Barb Wire" starring Pamela Anderson and 1992's "Tank Girl" were no great shakes either. So much for "girl power" in comic books.
And let's not forget "Steel," loosely based on a "Superman" spin-off starring Shaquille O'Neal!
Then there's "Judge Dredd" w/Sylvester Stallone, "Supergirl" (1984) starring Helen Slater, 1997's godawful "Spawn"...I could go on and on but I'll let someone else nominate a few!!!