Having never tried it, but going by the reasonably glowing recommendations on this site, the clove oil method seems to be the fairest. I have read several threads today (in case I ever have to do this) and the clove oil method seems to be the fairest. If the testimonies are true, the fish involved literally falls asleep, under no panic or stress, and never wakes up.
Having said that, the cold water (I didn't even use icy water!!) method seems just as instant. I have done water changes on a sick tank of fish, and although I used dechlorinator, the colder water going in caused 4 rasboras to literally just belly up instantly and never move again (I was as green in this hobby as a 2 inch thick algae coated tank at the time). So, although I have read some rather alarming reports from people on the ice water method, in my VERY limited experience, it seems a valid way of handling this problem.
I did wonder though, car anti-freeze will lower the temp of water quite a way below 0c, and remain liquid, and is quite toxic too to humans, so surely it would be incredibly lethal to smaller fish? Any experts have any comment on that?
To the OP, I would feel personally that it is better to do something however unpleasant it may be sooner rather than wait for an acceptable method to come along only for it to be too late, it would save the poor fish some degree of suffering in the long term.
Like I said, these are just my thoughts, not authoratitive advice.
EDIT: Before anyone jumps all over me about my comment regarding the rasbora incident, firstly I was (and am) NOT proud of it, and secondly the water wasn't especially cold, it was a 40% water change with the heater on and the water was approx 28c in the tank, and approx 15c going in. Yes I now know the water going in should have matched the tank, but like I said I was incredibly naive when this happened and I now regret it.