Quickest, most painless way is the beheading. Just use a large knife with some weight, like a machete or cleaver. Cut the head off just behind the gills, the head will die almost instantly. Alternatively, you can freeze them. That is a time honored method. With fish, their metabolism is dependant on the ambient temperature. If you lower it, they go to sleep. They will die long before their cells start to crystallize. It does take a while though, and how de we know it doesn't really hurt them? With smaller fish, I used to throw them as hard as I could against a concrete wall. Was nasty, but very quick.