For starters, pretty much anything with Nicolas Cage is goddamn comic gold. The Wicker Man is an obvious one, but if you haven't seen the bat-shit crazy "Drive Angry", do yourself a favor and go rent it.
There was a scene in "The Phantom Menace" where Anakin Skywalker is in bed and having a nightmare, and it seriously looks like he's receiving a terrible blowjob. I know Hayden Christensen is a terrible actor, but I remember the whole audience at the movie laughing hysterically at what was supposed to be a serious scene.
About 50% of the 1990's "Judge Dredd" film was unintentionally hilarious, but one scene in particular made me and my friends laugh out loud: after Dredd kills the Angel Family and his mentor gets killed, there's a part where he's looking at what I assume is a Virgin Mary statue in the church for about a good minute, and all my friends could think about was that this statue looked like it had a 38DD chest. Seriously, it looked like 90's Pam Anderson in statue form.
And basically any scene with John Travolta in "Battlefield: Earth". Actually, just "Battlefield: Earth".