Soviets, USF, Ostheer and OKW all seem Viable, brits I didn't see enough of yet.
For Brits and Ostheer you really need a well developed strategy though, I felt I did best with ostheer, but I had practiced Ostruppen alot.
VonIvan was using Mech Assault for which he has a really well practiced build
I saw tightrope was doing a standard 4 grens mg, even with a relatively heavy t1 and fine play he got pushed off in that 5-7 miniute range when Ostheer tends to struggle to counter vehicles, infantry and mgs at the same time.
Edit: I really want to see more of the top seeds play ostheer to try to improve my own play, but many are choosing OKW