Monks are, along with Warlocks, my favourite 5e class.
My personal fave Monk sub-class is the Way of Mercy (which might be heavily driven by something I’m looking to do in my campaign), with the potential for that to be replaced by the Way of the Ascendant Dragon that’s currently in Unearthed Arcana for playtesting and totally isn’t in any way going to show up in any sort of Dragonlance book.
Your choice, the Way of the Long Death is really, really good too, but it has one of the most broken 17th level capstone abilities, allowing them to do up to 20d10 of necrotic damage (which not much in the game, outside of the undead, is resistant to). That’s just brutal. Granted, they get a Con save to potentially half the damage, but still...
And if you took a two-level dip into fighter to get Action Surge, you could then do it all again with your remaining Ki points in the same turn. So by Level 20, you have the potential to be dealing out up to 360 points of necrotic damage in a single turn. And given that monks tend to be high Dex, they’re often acting first in combat, so they could absolutely kill the fight before it starts. Then they can just sit down for a short rest, get all their Ki points back, and do it all again in the next fight.