Regarding Naval Units, some general rules…
SUBs are the cheapest and best for offense AND defense against enemy NAVY. They are worthless (except as fodder) against AF
TRNs are the same price as SUBs, have a minimal defense against enemy navy AND air force., but are only fodder on attack. A TRN’s real threat is carrying land troops…
DSTs are a good all-around buy. You NEED at least 1 if you face enemy SUBs since they nullify the “sneak attack” AND submerge options. They are also very good at both offense and defense.
ACs are great defenders, especially if loaded with AF. Buying loaded AC’s is expensive, but if you have available land based FIGs to land on it, it is a very inexpensive way to create a MASSIVE defense for your fleet quickly and cheaply.
BBs are the shock absorbers. By soaking up the first hit, they are valuable against subs, AF, or any other attack on your fleet. They are powerful on defense and offense, and the shore bombard makes your TRN’s that much scarier. But, because of cost, most nations never buy them, they just have the ones they start with.
BBs alone are MASSIVE targets for enemy forces.
SUBs alone are screaming for enemy AF to hit them.
TRNs alone are screaming for enemy navy and AF to hit them
Carriers alone are at SERIOUS risk to enemy SUBs, especially if in open water (the FIGs are lost if the carrier is sunk)
FLEETS… not single ships.