Depends on what you mean by easiest. In terms of physically easiest, I think shooting yourself (if guns are available). It's my preferred way, but I really don't want someone to have to find my head gored by a shotgun. I think to a certain extent it's psychologically easier because your chances of living if you shoot through your mouth are... basically none. I can't imagine the hellscape of surviving an attempt, under any conditions, and that's what would stop me from trying.
All that being said, if you plan to CTB by gun, I think you should take some gun safety courses (lol) to make sure you're comfortable with the mechanisms of the weapon and shooting it so you can be confident when you go through with it.