By BST please. I'm pretty sure the strongest legendary is Eternatus, so I don't need that.
Well, let's see.
The strongest mythical Pokemon is Arceus, with a base stat total of 720. All Pseudo-Legendaries have the same base stat total. The strongest legendary (non mega) is Zamazenta and Zacian Crowned, if you don't count em, then you have Zygarde complete, and if you don't count them, you have kyurem-w and kyurem-b.