Imo it's something along the lines of:
Sov/sov > sov/usf > ost/okw > usf/usf > okw/okw > ost/ost
I have started playing 2v2 (3v3 was fun but either too easy or too hard and depended too much on teammate's skill level) as the OKW and this is pretty much what I have run into. Although the hardest I think is Sov/USF, only if the USF player has the rifle company doctrine (which is free right now ). It is especially difficult if the Soviet player has either a doctrine that can call in shocks or guard rifle and is-2 or t34/85s. Really hard for double okw to beat. Massed, spamed, doctrine medium tanks from the allies I think is the best strategy in the meta right now. Two E8s and two T34/85s can come out before the OKW players have even one panther.