I have several, but whenever someone mentions guilty pleasures, Event Horizon is one of the first ones that pop into my head. While no masterpiece the film is incredibly well shot and made an impression on me when I was younger. And it's a lot of fun in a certain kind of way.
I don't feel guilty about loving Event Horizon, that movie is a sci-fi horror classic.
Now Valerian and the City of a Thousand Planets, that's a guilty pleasure for me. Terrible main characters, terrible chemistry, script is something to be desired, but man I will watch the shit out of that film any chance I get.