In most cases, you just want to lose in order from the least valuable unit to the most valuable.
If you are the defender and you have 1 destroyer and 1 cruiser
The attacker has 2 subs and hits you 1 time
If you lose the destroyer, you are vulnerable to a surprise shot during the next round, and you would not get a reply, so losing the cruiser before the destroyer might be the smartest move.
You have 3 bombers and 2 infantry. Enemy attacks you with 2 fighters and 1 bomber and hits you 2 times. You may want to lose one or more expensive bombers (defend;1) before infantry, in the hopes of keeping the better defenders alive now to drive off the enemy as the combat rounds go on. Once all the bombers are dead, however, the enemy will probably just leave.
AAA guns cost 5 but they can’t fight after they do their special shot, so lose them first no matter what.
Carriers are weak attackers, and expensive, but in some editions they do have 2 soak hits. However, if you lose or damage a carrier first, they cannot collect planes after the battle, reducing your defensive posture and possibly giving your planes no where to land. While Battleships should always soak a hit first, carriers probably should not. Note that losing carriers first keeps your combat power high (attack 1 or 0 lost vs losing a cruiser or fighter at 3). It can be tempting to dump the carrier first, but you may regret this after combat is fully resolved…