All-time-favourite in the category:
"Bad Boys II"

Also:
"Pearl Harbor"
"The Descent"
"The Legend of Zorro"
"Fun with Dick and Jane"
"Rumor Has It"
"The Dark"
"Matrix Reloaded & Revolutions" (though they weren't particularly bad ... just not up to the first movie)
"Dreamcatcher" (what an aweful movie-experience)
"Tomb Raider" (the trailer was at least intriguing)
"Men in Black II" (one of the worst movies in years)
"Mindhunters" (likewise)

I'll update the list as movie-titles spring to mind .

Best regards,
VSG