Legendary monsters hold a special place in 5th. Battles with such monsters are meant to be special, often boss battles that are the endings of arcs or even entire campaigns. Legendary monsters are supposed to provoke awe and dread, and a battle with one should feel like a truly epic moment.
So if you consider a combination of flavor (how does the monster feel, how does it affect the campaign, etc) and mechanics (how well does the monster handle a party of PCs, and how interesting does it make the fight), what do you think is best exemplar of the "legendary monster" in 5th edition?
So if you consider a combination of flavor (how does the monster feel, how does it affect the campaign, etc) and mechanics (how well does the monster handle a party of PCs, and how interesting does it make the fight), what do you think is best exemplar of the "legendary monster" in 5th edition?