Best is a tough pick, I don't think I can do it.
But there are some important worsts!
Plan 9 From Outerspace: So bad, it's hilarious. You can see the strings holding up the flying saucer, and it comes with the nuclear proliferation speach that was inevitable at the time. Plus, it was the last film to ever feature Bela Lugosi (The original Count Dracula), because he died half way through the filming, they replaced the actor, but never reshot the footage he was in.
Alone in the Dark: Don't even get me started, so bad it's not funny.
Halloween 3: (my personal favorite bad movie). You know how Halloween and Halloween II featured derranged psycho killer Michael Myers? Halloween III didn't! In fact, he never even came up. In his place was an evil Irish toy company, staffed by androids. This evil toy company steals Stonehenge, hooks it up to a big laser thing, and uses it to mass produce cursed Halloween masks. These masks are sold to small children, and on Halloween night a special commercial is played which makes the masks turn the children's faces into goo, snakes, and spiders. And to top it all off, the main character looks like Magnum PI.