I'll have to go with Russia too.
The US has been more competitive lately, but their style is too similar to ours.
Our rivalry with Russia has always had the element of a clash between different hockey schools.
Sure, it's a bit archaic nowadays with some Canadians playing European-styles and some Russians hitting everything in sight, but I think the element of two worlds still has added spice.
The American rivalry is not really seated in hockey, and is simply part of our complex with respect to our southern neighbours which extends beyond sports.