I think that in most WW2 games most ppl want to play the evil Nazis. Not sure why. In COH2 a big reason is probably the fact that the Germans have all the famous tanks. Tigers, Panthers, Kingtiger, etc.The reaon I prefer playing Germans is simply the design of having more specialized, yet more powerful units. I hate throwing away medium tanks by the dozens or rushing blobs into MGs.
Also Soviets are way too restrictive: Either you go for Advanced medium tanks OR heavy tanks. It's possible with one doctrine to have advanced mediums and heavy tanks but then you lack Elite infantry. And so on.
The quality factions are OKW, Ostheer and Brits. OKW because of the additional vet and Ostheer because of good specialization. Why don't I like playing Brits then? Because those are way too easy and cancerous (Comet, Mortar Pits, double Brens).
The most fun and rewarding faction is Ostheer since it has everything available and requires you to combine your arms and think.