A really touchy subject that sometimes gets really ugly.
Having said that, There is no "best" way IMO. It's the method that works for you the best.
When I have to I put then in a ziploc bag as fast as possible and slam them on the floor hard enough to insure instant death.
Hard to do ?
As a fish lover, yes. Physically, no.
Once committed, be sure to do it right.
If a fish survives the "slam" method, then you were not commited in the first place, so don't do it. Use other methods that suit your conscience better.
Centrifugal force is a powerful thing.
I've never had a fish survive the first shot, no matter the size.
Bigger the fish, bigger/faster the swing.
Sorry to sound cruel to some, but it's easier for me to get it over with, than thinking I have a fish dying in the freezer that might live if I go get it out.
Guess I'll need to station a fire extinguisher close by now.