None of them are OP to me. Each one is a careful decision of rock-paper-scissors, and if you try to just keep using rock every time for every battle, you will always lose against an active opponent. The spies provide a small bonus to the effectiveness of your military, but don't really serve to do much beyond getting a very slight edge if you are evenly or outmatched. You want to use ground if your enemy has air, you want to use navy if your enemy has loot, and you want to use your airforce to remove their airforce and take out their next most powerful branch, army or navy. Missiles and nukes are only so effective, and they don't even affect military units, so really you could only use them to intimidate someone into peace with the threat of collateral damage to their nation. No part of it is OP, and it is perfectly balanced, as all things should be.