Milita is a really good unit for defense, also RRG and heavy tanks. Keep milita in cores and rail road guns in your core provinces and heavy tanks to slow the enemy down.

The best defense is a good offense.

A simple concept for a defensive stack is to have a stack of ranged units with lots and lots of anti-air with it. Then include armoured cars, which kill unarmoured units and scout for the ranged units, and add something the kill tanks. Anti-tanks or tank destroyers are the most preferable, as i dislike heavy tanks a lot.

This stack is really tough to beat but isn't very mobile unless you detach the ACs and/or TDs (if you have them). It can go on the offensive if needed. A major weakness is if the enemy brings superior firepower or if they use railroad guns as their ranged units but you do not.

True, but sometimes I might need to advance on one front while defending my rear before launching an assault, like the von Schlieffen plan. It's not good to only defend, but it can be needed to buy time.
Although it's cheaper to ally or have non-aggression pacts with neighbours instead, however temporarily.