Lethal injection would be my choice, or being shot in the head...rather than being hanged or beheaded.
I was watching some History Channel program the other day, and what I found quite interesting that I didn't know about hanging, was that hanging is intended to snap your neck and knock you unconscious, and once it does so, then you choke to death. That seems quite humane to me, but then again, your head can come clean off and you can also strangle to death while still conscious. I suppose nothings fullproof...