Over the years, I have thought many times about the process of being executed. One outcome of this is that were I given a completely open choice of how to be executed, assuming that I knew that there would be no alternative, I think I know what that choice would be.
My top preference would be to be hanged by a properly conducted long drop. With a long drop, there would be relatively little time between being restrained and being unconscious and dying.
Over the years, my second choice has drifted, but I would probably now settle for lethal injection, but by just a bit. Lethal injection has the drawback of a very long restraint period, which frankly scares me more than any pain associated with the execution itself.
Just below lethal injection would be electrocution. The restraint time for electrocution is a bit more than for hanging, depending on the exact procedures, and the time from the start of execution to unconsciousness is also short. But there is a higher likelihood of pain.
Next would probably be the gas chamber. This is interesting since gas chamber executions are probably the most likely to cause suffering, and leave the commended prisoner conscious the longest. But, they are sure and within their way reliable. Possibly more reliable than lethal injection.
Next would probably be beheading, which is at least quick.
Of modern, or relatively modern, methods my last choice would be to face a firing squad.