If you speak civilain unit I would say it it Longyearbyen (ENSB) in Swalbard ( Spitsbergen ) at 76 N . Been there, pretty cold in winter.
If you talk military I would vote for Thule AFB in Greenland.
Alert ( Elesmere Island ) is a bit further North than Thule, but I do not think they have an ATC unit as such there ( any Canadian friend can correct me of course)