Mickey Mouse. Easily. The sheer omnipresent nature of the Disney Corporation in
everything for decades seals that one without any contest. Nintendo does video games and not much beyond that, he's an icon to you and me, and to my parents, but my grandparents don't give a fuck about him.
They know Mickey though.
Hell, think outside of the first world. Disney is fucking everywhere in India. Disney sells shit in China. You can find Mickey Mouse merch available for sale in flea markets pretty much anywhere that claims to sell Western toys.
Mario is confined to video games, and Nintendo more specifically. If you've ever heard of a cartoon, or Western media at all, you've probably heard of Mickey Mouse. He's also been around for a bloody century. He's older than World War 2.