The Ashbringer would have been the obvious option to be legendary, but Legion’s weapon design choice kind of screwed that up.
But Ashbringer wasn’t the only weapon to get hosed by that design choice.
The Doomhammer, Felo’melorn, the Scepter of Sargeras and Xal’atath should have all eventually been made legendary too at some point due to their shear power and history in the lore.