Monks in 5e get a bad rap. They are seen as weak and squishy. To those that believe this, I have but one thing to say:
You’re playing them wrong.
Like the rogue, the Monks can dish out a lot of damage, but are most definitely not meant to stand on the front lines. One subclass, the Drunken Monk, even handles this by allowing our flying fists of fury to flee without fear of repercussion. Any of the other many subclasses can handle this with a simple feat; either at 1st level as a variant human, or upon reaching 4th by any other race.
With its boost to speed, its handling of rough terrain during a dash, and the ability to disengage freely from any target you attempt a melee attack on, a Monk’s natural speed and multiple attacks can make them excellent at attacking groups then pulling back out of reach.
Some call this a feat tax, making it necessary to play the class effectively. I agree. Nothing in this beloved game is perfect. Min-maxxers, of which I am a recovering one, will theory craft characters from dusk til dawn. The number crunching is part of the strategy.
For those other players that are drawn more to the story, the various subclasses offer many themes and playstyles. Every monk gets some pretty good class features as they level.
In tier one (1-5) they learn slow fall, gain an extra attack, and can use their ki to attempt a stunning strike. In tier two (6-10) their unarmed strikes count as magical, they learn to avoid area of effects with their evasion ability, can choose to end being charmed or frightened with the use of ki, and are become immune to disease and poison. In tier three (11-16) they extend their ki touch the minds of others and can understand any spoken language and be understood by anyone with a language, they become proficient in all saving throws and can spend ki to reroll a failed result, and no longer need to eat or drink nor do they get weaker as they age. Finally. in tier four (17-20) their ki will allow them to become invisible for a short amount of time in which they are also resistant to everything but force damage, or they can even astral project.
The capstone is okay, in that when they roll initiative if they are out of ki points they regain 4 ki points.
In the Player’s Handbook there are 3 subclasses: The Way of the Open Hand, The Way of Shadow, and the Way of the Four Elements. You have your classic martial artist, your sneaky ninja, and your anime bender.
The Way of the Open Hand can knock foes prone, shove them back, or negate their reaction capabilities. They can use ki to heal themselves, between long rests they enter a meditative state that grants them the protection of the Sanctuary spell, and learn the Quivering Palm strike that is a save for 10d6 necrotic or drop to 0 hit points.
The Way of Shadow use ki to manipulate the darkness around them. They learn the minor illusion cantrip, as well can spend ki to cast several spells: darkness, darkvision, pass without trace, and silence. Teleportation from one shadow to the next also becomes a thing, and can blend in with them and become invisible. Taking advantage of the distraction they may also use their reaction to strike an opponent that was just struck by someone else.
The Way of the Four Elements grants them spell like ability that they spend their ki on. Various water, fire, earth, and wind spells are at their disposable, using ki to fuel them and increase their effectiveness as they level. This subclass is considered by most to be the weakest of all the monk subclasses. I haven’t played it myself, but I have allowed a player to use the water spells on alcohol. Fun is what you make it.
Xanathar’s Guide to Everything presents three more subclasses: The Way of the Drunken Master, The Way of the Kensei, and the Way of the Sun Soul.
Now Drunken Master brings to mind the classic Jackie Chan film. You act drunk, swaying about and making it difficult to land strikes on you. The acting is presented as proficiency in Charisma (performance). When you use flurry of blows your speed increases and you may also disengage freely. You can also quickly get back to your feet from prone, or spend ki to redirect a missed attack against you towards another creature. Later levels will see ki points being used to avoid disadvantage on saving throws, or a veritable storm of fists as your flurry of blows can strike five targets.
The Kensei is a master of weapons, gaining up to five weapon proficiencies that are considered monk weapons. These weapons can be used to parry, boosting your armor class when you make an unarmed strike, and avoidance is the ki (haha!) to a monks survival. You can also use a bonus action to buff your ranged attack’s damage until the end of the turn. The bonus is small, but as you level you’ll be able to spend ki to add your unarmed die to one strike per turn, much like sneak attack. The tier 3 ability allows you to dump up to 3 ki points into a chosen Kensei weapon granting it a bonus to hit and damage equal the number of ki spent for a whole minute, provided it’s not already magical. The capstone for Kensei allows you to reroll one missed attack with a monk weapon per turn.
The Sword Coast Adventurers Guide has only two new subclasses: the aforementioned Way of the Sun Soul, and The Way of the Long Death.
If you want to go channel Dragon Ball Z or Streetfighter then The Way of the Sun Soul is for you. You gain a short ranged radiant damage ranged which can substitute for any of your normal attacks, or by spending ki you can throw 2 more as bonus action. As you level this evolves into the ability to cast a radiant vewrsion of burning hands and a fireball, all at the expenditure of ki. Finally you can light up with an aura that does retaliatory damage to those who strike you. Shiny!
The final subclass here is The Way of the Long Death. Obsessed with death and process of dying they have learned an innate understanding of mortality. When they defeat an opponent they gain temporary hit points that scales with their level and wisdom modifier. As they grow in power they can inspire fear in any creature within 30 feet until the end of their turn, but it requires their whole action. This will affect allies as well as foes, so I’d probably just dodge rather than risk your friends having disadvantage as well. Their tankiness comes into play at 11th level where they can spend 1 ki point to come back from 0 hit points with 1 hit point. Their final power allows them to spend up 10 ki points and force a target to make a Constitution save or suffer 2d10 necrotic damage per ki point. Even with success saving for half, this is still pretty awesome.
As future supplements come out, I’m sure there will be colorful subclasses, but these 7 presented here are a pretty good place to start.