Trick question, we all know the real answer

I honestly think the covid factor is somewhat overstated; the lockdowns likely added a few million to its peak, but it still expect it would have topped 150m without the pandemic.

This.  It sold because of convenience.  The world has moved to convenience via Spotify, Steam, Netflix, Amazon, etc.  Nintendo capitalized on the convenience trend with the S1.  

Definitely BOTW. Without that killer launch title I think things could have been very very different.

I think you guys are underestimating the number of casual gamers and parents that see a Mario Kart ad on the TV and decide to buy a Switch for their children

MK8 did actually make the WiiU sell better the sales were significantly lower before it arrived, on launch WiiU shifted like 300k that week and the baseline for sales increased so it is very much a platform seller, the platform itself just couldn't be saved.

BOTW

The Switch proffited from many things at once. Being hybrid meant that it was the only dedicated handheld at the time. Joining together Nintendo's handheld and home console output into a single machine also gave it a greater library at a very decent pace. And the first year had all of the WiiU ports everyone wanted to play, but noone wanted to buy a WiiU for (BotW as a launch game was massive). It also got extra sales from the Covid boost. From there, it just went up and up.

You have to start at the beginning and BOTW brought the confidence to Switch at its launch that was maintained by MK8 Deluxe, Super Mario Odyssey, Smash, Pokémon, Animal Crossing, and so on.

While not a singular game I would argue the Pokemon titles are up there, as there seems to be a substantial fanbase who will buy almost any console to play the series.