The Room. I saw it on Adult Swim on April Fools Day 2009.
I must say, I hadn't laughed so hard in my entire life.
http://www.imdb.com/title/tt0368226/ - In Case You think you've found it but you aren't sure (Believe me, Tommy Wiseau didn't grab an original name when he called it "The Room")
The movie also has great quotes. I must say I love a lot of movies from great directors, but Wiseau isn't a great director, but this movie is probably the best Bad Movie I have ever seen. Sex Scenes that are so horrible, Ron Jeremy shudders when he watches them. Dialogue and acting is so damned atrocious, that's what makes it so funny.
Please watch it, it is amazing. If it doesn't change your life, it will at least change your perception on movies.
And the Next Time you think you've seen a bad actor... just think "The Room". Below is the trailer, but trust me, it's much funnier when you see the whole thing. It's the "Rocky Horror Picture Show" of my time.