I still vote Ether on this one, It used to be an anesthetic. I've heard from PEOPLE that had it used on them back in the day and they say you feel warm, then kinda hot, then nothing. You don't suffocate them, and it's over quickly.
I had to use this on a full grown hen recently after a coyote punctured her lung. She was gone within about 15 seconds. She did flap around AFTER she passed (muscle spasms, you typically see this in every animal) but her actual passing was calm. I did make sure I had a stethoscope on her to make sure. I used to work for a vet, so I like to be clear.
Everyone has different methods. Everyone has different views on what is humane. Calling people psychos because they care enough to ask and offer is pretty rude. IMO--freezing is about the least humane, next to letting it sit there and suffer. THAT is painful and cruel to me.