Plan 9 from Outer Space (1959)

The epitome of "So-Bad-They're-Good" cinema, this is often called the worst movie ever made: with its incoherent plot, jaw-droppingly odd dialogue, inept acting, threadbare production design, and special effects so shoddy that they border on the surreal. But it's an oddly endearing disaster; boasting genuine enthusiasm and undeniable charm, it is the work of people who loved movies and loved making them, even if they displayed little visible talent.