Short answer: whatever makes sense.
When in battle, you generally don't just strike at random foes. Depending on the combatant's training and role they will choose different targets. Each creature you decide to send into battle should have some preferred tactic to utilize and as the DM, you get to decide what tactic makes the most sense. Some examples:
A mother bear protecting her cubs
The party wanders upon a mother bear and her cubs. The mother will attempt to place herself between the players and her cubs. She will target whoever is closest to her cubs as they pose the most threat.
An enraged orc warchief
You've fought through the orc camp and find their warchief in a bloody rage. He does not care who he hits as long as he causes pain. He will charge whoever particularly hurts or offends him in the previous round and will attempt to finish off any enemy showing weakness.
A trained sniper cripples the party
An assassin has been sent to dispatch your party. The first thing he does is try to incapacitate your cleric so that nobody will be getting back up during the fight.
A cornered scoundrel fights for his life
You've cornered that pesky assassin after his plan failed and he's become frantic. At this point his attacks become more or less random, lashing out in all directions just to buy himself some space. A more level-headed character may attempt to create an opening through which to escape.
Try to think of combat as a social interaction, each character has certain goals and talents they can use to reach those goals. The tactics used by each creature should fit its personality, training, and objective. And as I demonstrated in my last two examples, circumstances can play a major role in these tactics; dynamic tactics make combat interesting and keep your players on their feet.
And a short note on theater of the mind. Rather than think in grids for combat, think about who the major threats are to your creature. For example, if your creature will merely attack whoever is closest, keep track of who is closest, second closest, etc. to the creature. If they are easily enraged, think of whoever has done the most damage to them. Theater of the mind focuses the battle away from numbers and more towards motivations.