For me, it’s probably artillery. As long as it’s out of range of direct combat, it’s really useful for defending against stronger nations, especially if you’re mountainous and/or arctic.

Tanks are also really good, and are basically the powerhouse of your military early game. It’s not as good as artillery however, as they’re really weak against infantry if they have anti-tank technology.

Infantry is really good, and is definitely quite useful for mountainous terrain (before the mountainous specialization update). They can also easily defeat tanks if they have anti-tank technology, but they’re pretty weak early game compared to other units.

Anti-Aircraft isn’t bad per se, but they’re pretty useless early game and in some cases, even mid game. Aircraft are pretty overlooked in RoN, so there isn’t much use for them. It should also be mentioned that destroyers and fighters are more effective against aircraft in certain scenarios.