What's Nintendo's best expansion/DLC so far? (and why)

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Nintendo have added expansions to many of their games over the last few generations, from Xenoblade's Torna the Golden Country and Future Redeemed, to Bowser's Fury on the Switch port of Mario 3D World, to more recently the Star Crossed World content in Kirby and Forgotten Land on Switch 2.

What is their best such expansion to date in your opinion, and why?