ORIGINAL: 76mm
I'm playing the AI normal/slow right now--it is really beating me up in September, when we don't get any new armies. Hopefully I can hold on a few more weeks...
Key to riding the dry spell out is defensive posture, terrain, and fortifications.
Very easy to stop the Axis cold up north. In the center I was counterattacking from early July on and gradually burned out AGC.
The south sucks. Was seriously considering diverting forces from the center for a while and taking the PP hit for doing so. You just don't get much down there in relation to the space covered and it's wide open. Even defensive posture doesn't make a huge difference.
Blowing the bridges on the Dnepr helps some, but they'll be in your face regardless.
I think against a human player you need to build forts down there. I put up this magnificent fort line in front of Moscow that turned out to be totally unnecessary.