Mario Kart 8 Deluxe, and it's not even close.

I really enjoyed Torna, Bowser's Fury, and the MK8 Booster Course Pass, but my vote has to go to Future Redeemed.

It wrapped up the Xenoblade trilogy nicely and I found the foundational core gameplay and tone of 3 to better than 2.

So many to choose.
Torna was released as a physical copy separate from Xenoblade Chronicles 2 as an option, so it almost feels like cheating.
Mario Kart 8 and Mario Kart 8 Deluxe with its Booster Course gave some pretty peak content.
Animal Crossing: New Leaf added a lot with Welcome Amiibo as a free mega update.
Animal Crossing: New Horizons has added a ton of great expansions and Happy Home Paradise is probably the weakest despite it being the paid one.

DLC so good it made me scream and stay wide awake for like 30 hours.

Very cringe I know.

Not cringe, I used to stay up til like 2am to watch E3 back in the day and then be so hyped from the reveals I couldn't sleep and pulled an all nighter.

Nothing wrong with being excited about something.

Hey look, not to end the "debate" buuuut Torna the Golden Country is such a massive expansion and extension to Xenoblade 2 that they actually released it in a physical standalone version !

But seriously, Monolith Soft is a god at doing those.

Other big expansions/DLC to mention that I really like would go with the Mario Kart 8DX Booster Pass, Bowser's Fury in Mario 3D World, Kirby and the Forgotten Land Star-Crossed World's and none other than Future Redeemed from Xenoblade 3 to cap off my thoughts on it.