Naively, the colder something is, the faster it could freeze a person. However I’m told that liquid nitrogen for example forms a vapor barrier which actually slows heat transfer. And then there’s the specific heat of the coolant, which can vary a lot. Liquid helium is very cold but has a poor specific heat.
I presume then that the fastest freezing would occur when immersed in the coldest possible liquid that doesn’t boil at body heat. I did some research on this awhile back and came up with N-ethyl-N-methylethanamine, which at standard pressure boils at 60C and requires an astonishing -196C to freeze.
At that point it would be a question of heat conduction through a body: how fast heat could escape the body core; that I haven’t a clue about.