There is a small argument to be made here that because Breath of the Wild is a very different Zelda game that appeals to a much larger audience that it has a chance to challenge Odyssey.
That said, Mario is Mario and Zelda is Zelda. Zelda is a game and Mario is a brand. He's the jam to Nintendo's peanut butter. You don't see Nintendo without seeing or thinking Mario(and more recently Pikachu). On top of that, Nintendo will market Mario games much more, as he is their home console killer app.