Here's a list since it's impossible to pick a single most-corny movie.
"Smokey And The Bandit". That movie seemed to be little more than a trucking industry propaganda film.
"Chopping Mall". The package cover shows a serial killer holding a shopping bag full of body parts, but the movie was about mall security robots running amok.
"Flush". A slightly similar plot to the movie "It's A Mad, Mad, Mad, Mad World", but very low budget, and they actually inserted cartoon sound effects over every spoken swear word.
"Nail Gun Massacre". One of the shittiest horror movies of all time that actually contains a scene where an actor is clearly reading from the script, and a "dead" man moves to reposition himself.
"Starship Troopers". 'Nuff said.