Hey all, been DM'ing a while but would appreciate advice on how to choose which PC monsters should attack by default?

Currently I favour option 1 ("most recently attacked") of the poll, but grateful for any thoughts!

NB for voting, 5 = 1st place, 4 = 2nd etc

I have smart monsters attack based on what they think is most likely to let them win (often this means eliminating apparent glass cannons). Stupid monsters generally attack whoever hurt them the most since their last turn.

Quote from Pantagruel666 >>

I have smart monsters attack based on what they think is most likely to let them win (often this means eliminating apparent glass cannons). Stupid monsters generally attack whoever hurt them the most since their last turn.

This is what I do. And in the first round of combat I roll at random to see how stupid monsters attack. 

A lot of the time it is simply who is in range

I like targeting to be fluid based on the action. 

• A group of goblins (three swords, two archers) might be focused on the barbarian and cleric of a party when the quiet wizard in the back of the room suddenly unleashes a barrage of magic missiles that immediately drop the goblin's best swordsman. The two goblin archers immediately switch target to the wizard and the party must then also react accordingly.
• In the heat of battle a troll is trading blows with the fighter of the group when the rogue suddenly emerges from behind with a brutal flaming rapier backstab. The enraged troll then spins around in a blind fury to try to crush the source of the devasting blow.
• A group of elite Drow assassins immediately start the battle by flinging their poison daggers at the party's cleric, knowing that dispatching the healer first is their most potent strategy.

Taking the intelligence of the opponents and the circumstance of the battle makes it much more immersive and also brings in an element of strategy to the fight. When your best fighter suddenly has to fall back and hold his tower shield in front of your sorcerer as he dumps out ridiculous loads of damage then you now have something much more than your usual hack and slash encounter.

Eh I don't really use a system like that. There are optimal strategies to be sure, but how irritating it must be to have a well-crafted tanky fighter just for every monster to go after the squishy wizard. I know this has been litigated elsewhere, but DMs who aggressively optimize attack placement make me wish there were more forceful tanking options for 5e.

There's really no way to answer this, since it depends entirely on the monster's intelligence and motivations

Simply put, it depends on the monster and its attitude, abilities, and goals.

An owlbear might attack the nearest PC to her cub, for instance. A mindless zombie might attack whatever happens to be in its path. A sahuagin will favor PCs who have less than full hit points because of their blood frenzy festure, and a gnoll will favor the lowest hit points in order to proc their rampage feature. Intelligent friends and undead might go after the cleric or paladin because they have the best chance of cutting through their defenses, or they might avoid them for the same reason. Kobolds, assuming Forgotten Realms lore, will go after gnomes with a vendetta.

These are all just general examples and suggestions, of course. Just try to think about what a monster wants and what tactics are favored by its abilities, and you'll have a pretty good idea of who it's most likely to target. A great resource for this is Keith Amman's blog (and his book by the same name), The Monsters Know What They're Doing.

There are a lot of factors to unpack here. How intelligent is the monster? A beast will likely attack whoever is closest or whoever hurt it last unless defending a young one is involved. Smarter creatures should fight smarter.

It would always depend on the monster:

Are they a beast that is hunting for food? Kill the least armoured or most injured and drag it off.

Are they a raiding party of intelligent creatures?

Are they defending their home?

For me I base it off what they is fighting... some monsters don't have high intelligence, some don't have free will... if the set up is like that I'll either attack the closest or I'll roll for it.

The more intelligence, tactics the monster would have than the system plays that way... I never just target characters to target them... unless their is a story reason why... but that's different.

Whichever will create a dynamic encounter that makes sense.

I target whichever character whose player I don't like. I'm kidding, of course. Many previous posters gave good answers that I won't repeat. I'll give examples that haven't been discussed yet.

• I've had NPC allies give priority to defending PCs they favor. One could have a crush on a specific PC, so they'll go out of their way to ensure their survival. Sometimes an NPC may be experienced and smart enough to know which monsters should be prioritized, and which party members need protection.
• If my players are smart, they can easy manipulate an ogre to attack the tank instead of the glass cannon. Whoever insults him gets priority. That's part of the lore. Bonus points if you're leading him into an obvious trap that he's too stupid to notice.
• If one of my players playing a bard thinks it's a good idea to try and seduce the evil dragon, I have them make a charisma check. I don't care if it's a natural 20, the dragon will just roast them as their answer. In-game, the dragon is unamused. Out of game, I'm teaching my players that's not how the game works. Perhaps if they roll a 30, the dragon might find the bard amusing enough to spare them and keep them as a slave for entertainment.
• I've had a doppelganger use mind reading to know more about the PCs before engaging them in combat. That's basically a free pass to impart DM knowledge to a monster in order to improve its tactics in combat. If for example the doppelganger knows that the PCs want him alive, he may act more recklessly.

It varies based on the creature motivations and the circumstances of the battle. 

• If a monster is just in its home resting when attacked it will probably lash out at who ever hit it and flee if it gets hurt too much. 
• Intelligent creatures planning to kill the party attack the weakest  characters and the spell casters first
• Hungry creatures that want to eat target the least armored and most injured  and flee if hurt too much  or if they get their prey. 
• Intelligent creatures with specific motives will target people that threaten those motives such as going after a mcguffin or alike. 
• A dumb monster like an automaton likely targets the closest thing to it or may have a specific rule it follows  like a robot.

Smart players can effect this by disguising caster, faking  or hiding; or possibly goading the target. 

If they always act the same players will pick up on and instinctively abuse the patterns

I didn't vote, it completely depends on the monster.

A lich will behave completely different from a bear.

Even intelligent creatures have different behavior, is the creature passionate and will go after the PC that has angered it the most? Or is it rational and clever enough that will take out squishes/glass cannons?

For monsters with > 6 INT- whoever I would attack on that situation 

It depends on the monster and the situation, but as a DM who tries to avoid killing players, I often attack whichever PC can handle it.

A lot of people are already saying similar things but I feel the monster would most likely attack easy-to-hit or low-hit-point PC’s. They can see which ones are the squishy wizards due to very obvious differences in equipment such as lack of armour, and will attack accordingly. A large number of the monsters in the MM are just animalistic hunters, and would probably not attack someone who is more noticeably armed than the spellcaster in question. More intelligent monsters would more likely try to kill whoever has the highest damage output, whichever character they believe is leading the party, anything to gain a tactical advantage. Monsters with an intelligence of 21 or higher would often be able to tell what classes the players are and what abilities they have simply by looking at them, and would respond accordingly.

Quote from BasementBoys87 >>

A lot of people are already saying similar things but I feel the monster would most likely attack easy-to-hit or low-hit-point PC’s. They can see which ones are the squishy wizards due to very obvious differences in equipment such as lack of armour, and will attack accordingly. A large number of the monsters in the MM are just animalistic hunters, and would probably not attack someone who is more noticeably armed than the spellcaster in question. More intelligent monsters would more likely try to kill whoever has the highest damage output, whichever character they believe is leading the party, anything to gain a tactical advantage. Monsters with an intelligence of 21 or higher would often be able to tell what classes the players are and what abilities they have simply by looking at them, and would respond accordingly.

 I think that would happen at >12 or so

Quote from HeathSmith >>

Quote from BasementBoys87 >>

A lot of people are already saying similar things but I feel the monster would most likely attack easy-to-hit or low-hit-point PC’s. They can see which ones are the squishy wizards due to very obvious differences in equipment such as lack of armour, and will attack accordingly. A large number of the monsters in the MM are just animalistic hunters, and would probably not attack someone who is more noticeably armed than the spellcaster in question. More intelligent monsters would more likely try to kill whoever has the highest damage output, whichever character they believe is leading the party, anything to gain a tactical advantage. Monsters with an intelligence of 21 or higher would often be able to tell what classes the players are and what abilities they have simply by looking at them, and would respond accordingly.

 I think that would happen at >12 or so

 Certainly monsters with an intelligence of 12 or higher would be able to get an inkling of what kind of adventurer the PC’s are and formulate a strategy around that, what I’m saying is that monsters with intelligence of 21 or higher would have superhuman intellect capable of automatically knowing exactly what abilities and traits theses players have, as opposed to garnering a rough idea. Sorry off I wasn’t clear enough on that point 😅