Celsius, expecting a reading of 37 degrees. I never measure anything in Farenheit (I don't actually know how to convert to it, or what a body temperature reading in it ought to be). I do basically everything in Celsius apart from occasionally during my A-Levels (16-18) where I did some conversion to Kelvin.
A canadian once told me that in canada atmospheric temperature is measured in celsius but body temperature is measured in farenheit, or maybe the other way around.