>Rayquaza
It was the first one to start the trend

>Giratina
It was the first one to give them the ability to change forms

>Kyurem
The first one to have more than one form change and actually boost it's stats when changing form

>Zygarde
It was neglected for about 12 years before getting a game

>Necrozma
The first to start the whole "The reason why the gimmick exist in the first place"

Now for Gen 8, there are some debates on who is the true third legendary so, I'm adding both

>Eternatus
Currently the only pokemon with the highest bst of 1125

>Calyrex
For some reason, Calyrex is the only pokemon who's aura turns blue when Dynamaxing

>Terapagos
It's currently the only boxart and third legendary pokemon to have been given a gender. It's gender ratio is 50% male and 50% female.