I think the worst way to euthanize has to be baking soda, the fish basically suffocates, and you get to watch him do so.
blunt force trama is fast. I couldn't bring myself to use baking soda on an angelfish that developed swim bladder problems only about a day after I had bought him. However, I couldn't smash him either. My sister's friend had to do it while I was out of the room. It was a fast death. One smack and he was out. But it was definately tramatic.
that is my experiance for euthanisia. I am still for the practice, as I would rather have a swift death than a slow, painful one. I just don't like being the one doing it. However, it is less painful to watch a couple seconds of pain then it is to watch a fish slowly deteriorate and die before your eyes.