-27°C (-17F) with bicycle 5km to school. Yepp, the old times...
-18°C (0F) with 50cc 60km to office. Already first signs of global warming! Wasn't comfortable though.
-2°C (28F) with 500cc 820km (still one way!) to hospital (just this November). But that was easy, could have gone further without undercooling.
My comfort zone is somewhere down to about -5°C (23F). I have made many 300-600km rides at this temperature.
A friend claims to have ridden in -60°C (-76F) blizzard and to refuse glooves in any (always hot or hotter) European weather. Ok, he is an Alaskian expatriate, but still I want to see this myself. I know several Russians and they like to keep warm although they can withstand low temperatures.