My nomination is the 1980 classic, Flash Gordon. It has everything: Brian Blessed!1 Max Von Sydow! Topol! Timothy Dalton! Richard O'Brien!2 Sam J. Jones! A brilliant soundtrack by Queen! Beautiful, incredibly unique backgrounds! Boar worms! Ridiculously bad acting! Absurd dialogue! A fight in the form of a bizarre alien football game! This thing!

Just look at the poster:

It absolutely screams "Campy Sci Fi!"
Look how big Brian Blessed's mouth is!

Watch the video for the amazing theme song!
Read how much of a cluster#%£@ the production was! Screenwriter Lorenzo Semple, Jr. said:
[Producer] Dino [De Laurentiis] wanted to make Flash Gordon humorous. At the time, I thought that was a possible way to go, but, in hindsight, I realize it was a terrible mistake. We kept fiddling around with the script, trying to decide whether to be funny or realistic. That was a catastrophic thing to do, with so much money involved... I never thought the character of Flash in the script was particularly good. But there was no pressure to make it any better. Dino had a vision of a comic-strip character treated in a comic style. That was silly, because Flash Gordon was never intended to be funny. The entire film got way out of control.
Wikipedia
That's right - they weren't sure if the movie was a comedy or not, even while they were filming it. The producer didn't care that the main character was about as complex as a piece of cardboard. It was less like producing a film, and more like releasing a sack full of angry, drunken badgers into a kindergarten classroom just to see what would happen.3
But it gets even crazier than that! Sam J. Jones, the star and titular character, hated Dino De Lauretiis so much that he quit before post production began. A voiceover actor had to dub most of Jones' lines, and no one can remember who that voiceover actor was:
According to a 2012 interview in Maxim, Sam J. Jones had disagreements of some sort with De Laurentiis and departed prior to post-production4, which resulted in a substantial proportion of his dialogue being dubbed by a professional voice actor, whose identity is still a mystery.
Wikipedia
Can you even imagine a film production so screwed up that no one thought to write down the name of the guy who recorded most of the lead character's lines? Well, if you vote for this movie, you won't have to imagine it, because you'll see it for yourself!
Freddie Mercury explains why we need to watch this movie:
Flash is the Savior of the Universe
He'll save every one of us
He's a miracle
King of the impossible
He's for every one of us
Stands for every one of us
He'll save with a mighty hand every man, every woman, every child
Just a man, with a man's courage
You know he's nothing but a man, but he can never fail
No one but the pure of heart may find the Golden Grail
In light of this argument, I genuinely and emphatically believe that we owe it to Flash to watch this film. After all, he did save every one of us.
1During filming, the battle scenes had to be reshot because Brian Blessed was making "Pew! Pew!" noises every time he fired a weapon, and you could see his mouth moving. The director was not pleased.
2Best known as the writer-director of Rocky Horror Picture Show, in which he also played the role of the butler, Riff-Raff
3Seriously, it was ridiculous. The producer referred to Flash Gordon as "the only improvised $27 million dollar movie ever made". You can read all about it here.
4The article linked in note 3 describes Jones' departure rather differently: "At Christmas, he went to Los Angeles and never returned". The producer was proud that he was responsible for the only major studio film that was completed with the star AWOL.