There are several threads on this issue - do a search on euthanasia.
IMO the quickest way is the kindest, and without having vets meds / anaesthetic, then I believe that cutting the head off is the quickest. If the fish is large, then a quick whack on the head first - as I do when fishing is also probably a good idea to stun the fish.
Freezing is not a quick and painless option - the fish suffers as the cold penetrates its body, and in actual fact, crystals can form in the blood stream which i think would probably be excrutiating.
I've heard conflicing reports about clove oil, so as said I'd always go for the chop!
I understand that some may find it difficult, but its hard to argue that its not humane.