What Nintendo IP has the most system selling power now?

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Smash Bros is my vote. And I say this as somebody who probably likes it the least of Nintendo's big IPs

I put it over Mario Kart and Pokémon because it feels like those games have more overlap with other series and are less likely to be an independent system seller.

Smash Bros is able to use Nintendo's collective IP-base to get people into it. Also it introduces people to characters from their other games and sells those games for them. So many people were introduced to Xenoblade from playing Smash Bros, as an example.