Good two-nation combos

So, I’ve got to 3.7 in German Ground, 4.0 in Soviet ground, unlocked rank 3 in both USA and USSR air (forgot which BR), and currently grinding German air (Thanks for the advice, Noir89). Now, I planning on grinding 2 nations up to top tier (Air and Ground), because I want some variety. One nation I will “main”, and the other will be for when I’m tired of the first nation/ want to make money. Now here are my preferences.

Ground: I like firepower, flanking, speed. Also I would like some good Helicopters to use in GRB.
Air: I honestly don’t know what playstyle I like yet (maybe turnfighting), but I would like good jets.

What’s the best 2 nation combo that checks all or most of my preferences?

That’s always a bit of a tough question to answer as your goal seems to be top tier; if I was going to recommend 2 right now it’d be USSR and USA, they’ll typically always be relevant and are most likely to get new stuff throughout the year, they have solid lineups all the way through top tier.

Lower tier USA ground sometimes lacks in firepower but is typically about flanking and being mobile or peaking ridgelines/hull down play because of they’re good gun depression, at higher tiers many of the MBT’s end up very samey in playstyle between the NATO nations. As for helicopters, half the nations in the game use Apaches as their only option, so you may as well play the better Apache and have the option of using the AH-1Z, as well. And, for Air they have great CAS options all the way through the tree and the it’s really a tossup as to whether you think the f16c is the strongest jet currently; mig29’s have better missiles but the 16c has better flight performance and has a better CAS package.

USSR plays a little differently because your strengths are good armor profiles with small weakpoints and solid guns but your weaknesses are slow turret traverse, low reverse rate, and no gun depression. They have probably the best overall Helis in the game in the ka-50/52 and mi-28. Their aviation tree is a little lacking in good CAS options until later with the Su-25’s, the mig27k which are both very good and have particularly good rocket CAS, and even the mig29 can do some limited CAS.

I’d typically tell people to play what they think seems cool because the game changes a decent bit throughout the year and by the time you get to say top tier it may not be the same; though, USSR and USA and typically always in a good spot.