My response is for defence of non-core provinces. If you have them available, including commandos in your defending stack is great. If he has planes in the area, you might need AA in your stacks.
On mountains, your best counter would include 4-5 commandos plus armored units (commandos have less attack damage against armor), inf and motorized inf and art. You need greater than 80 defense points on mountains. I wouldn't fight with a big stack with less than 85 defense points. A diversified stack is better than a non-diversified stack. The more defence points the better your chances of winning. Better to hit with art and tac bombers to weaken them if you can. By having a diversified stack, you can lose more hit points before you start losing units.
In a city, you need a defending stack of at least 50 defense points. I would prefer to have at least 55 defense points. I would prefer a mixed stack of commandos, infantry, motor inf. and/or meck. inf.
In hills or plains, you need a defending stack of at least 40 defense points. I would prefer to have at least 45 defense points. On hills, you could add militia to your defending stack. They get a 75% defence bonus.
Art support is always great since you can get usually get in at least several rounds of artillery fire. Art needs to be behind the front lines when the enemy reaches your front lines.
If you can hit with an int/tac stack, that helps too.