Okay, worst movies that I paid cash money to see in the theater:
Caddyshack 2 - loved Caddyshack, what could go wrong?
Ghostbusters 2 - loved Ghostbusters, what could go wrong?
Leonard Part 6 - loved Bill Cosby, what... well, you know.
Howard the Duck was mentioned earlier - who the heck was the intended audience for that? I was 10 when I saw it and I thought it was ridiculous then.
And another vote for Batman & Robin, it just kept getting worse but it somehow never reached so-bad-it's-hilarious, it was embarrassing and painful to the end.