20F, because changing a flat in anything colder is too uncomfortable. I changed a tube one night in 18F and my hands got stupid cold, I cannot imagine fixing a flat on my non quick release wheels in anything colder.

I do not have quick release wheels and the flat was on the back of my Torker Graduate, which has 5 speed IGH and coaster brake. I may consider going colder on my fargo, but I dont have as good of lights on that as I do the graduate, and when it is cold, it is usually dark or about to get dark.