D&D 5E Agro and Target choices

[Garthanos]

5e doesn't have "aggro" or "taunt" mechanics. The closest that it gets is imposing disadvantage on targets other than you, to encourage enemies to attack you. Even the Compelled Duel spell - Which is the closest thing to a "Taunt" effect (because you need magic to do cool stuff) does not actually restrict the target to just attacking the caster.
It is still very much in the hands of the DM to decide who the monsters attack.

Even the cavaliers marking is decidedly not psychological because being distracted by the badass is not enough to affect you at all unless he is actually right next to you to reinforce it. Of course the battlemaster can focus fire one enemy into being unable to near him sort of the opposite effect desired ...

 

[Reynard]

Generally speaking, humanoid enemies in my games fight smart, choosing targets based on threat potential and using tactics to ensure victory, and attempting a tactical retreat when things go bad. "Monsters" I treat as hungry predators, more likely to down a target and then try and carry it off than move on to another opponent, and fleeing if they get too wounded or swarmed (watch some footage of baboons or hyenas running off big powerful cats like lions or panthers). Intelligent undead usually fight to the death because they are inherently evil and tortured by their state but choose targets based on malice. Demons, devils and other evil extraplanar creatures on the Prime don't fear death and are just horrible, so they inflict maximum shock and fear, usually willing to gang up on a target and coup de grace them if they go down so they can drag the soul back to Hell with them.

Dragons are a tough one for me. They are powerful and intelligent but they also care about their lives and treasure. If given a chance they won't fight the PCs at all but rather use them as pawns. In the case a dragon must fight, it will always focus fire to try and eliminate an enemy every round, usually starting with the biggest threat (which may or may not be the highest damage dealer, depending on the circumstances).

 

[jasper]

Player, "But But the monster manual says the magic eater would target wizards first, then magic items. Whimper whine."
Dm, "The wizard is fifty feet up on his broom of A10 war hog! You are prone, five feet from 6 of them and have 30 magic items on your person. And your portable hole so full if you sold the stuff at whole sale you would bankrupt 10 realms, 9 Greyhawks, and 3 darksuns."
****
D&D target selection should always be up to the dm. I know if they introduce common tactics, players would game the system.

 

[cmad1977]

I try to play the monsters as NPCs that have goals and motives. As such different creatures will behave differently.
I suppose for example,
The hobgoblins move up as one and methodically engage their enemies. They may not focus fire (or they might if given orders)and might leave an incapacitated foe alone and prioritize one that’s on its feet.
The wolves will attack from the side and back with a few of them bee lining for a particular target to overwhelm and are likely to continue mauling an incapacitated for.

 

[Oofta]

For me how I run monsters depends on the creature, their intelligence and level.

For example, wolves will likely make some feints and then go after the party member that separates from "the herd" or looks obviously weaker. But for the most part they aren't going to be able to tell the difference between a fighter and a caster.

A bear, on the other hand will likely charge whoever is closer, as will several low intelligence monsters such as ogres.

Some monsters are specifically militaristic such as hobgoblins are more likely to employ tactics such as targeting casters as part of their strategy.

Higher level monsters, especially those that either cast or have casters in their ranks will start targeting casters once it's obvious who they are.