Hybrid concept+Zelda+Mario Kart+Splatoon+Odyssey+dozens of neat small/medium sized games+excellent marketing is Switch's killer app.

The Switch is it's own killer app. The games are the bonus.

For now, the Switch has a killer first year. But if I had to say which game is selling the console, is BotW. Mario Kart is selling well too, but it's a port of the WiiU, Splatoon 2 has moved most of its sales in Japan, and Odyssey has come too late to move that many systems compared to Zelda, even with the holidays icluded.

Vroom in the Night Sky. It's a fresh and original game, we've never seen anything quite like it before.

If you want to play Zelda or Mario, just buy a cheap 2DS or SNES. This game on the other hand, can only be enjoyed on the Switch

BotW isn't a killer app for Switch because it's also on Wii U. Same with Mario Kart 8 Deluxe. It could be considered a Nintendo killer app but you're stretching what  a killer app is by assigning it as a Switch  killer app.

It'd have to be between Splatoon 2, Mario Odyssey, and ARMS.

And let's be real, it's not going to be ARMS or Splatoon 2.

BoTW was a big one, doesnt matter if its on WiiU cause not only few people have it but on 2017 absolutely nobody cared about the poor machine and its life was already ending.

And why on hell a game that sold 3.6m in less than 3 months like Splatoon 2 which is currently one of the 3 best selling games of the console cannot be considered as a killer app?

i know people are still being hit with the supply shortages and are still waiting to play Breath of the Wild. Mario Odyssey is a close second but Switch consoles are so limited at this point that people dont even have the chance to catch up. I didnt even have time to get hyped for Xenoblade 2 since Im still working on Pokemon Ultra Moon after bouncing off of Mario Odyssey. the number of amazing games Nintendo is pouring out is just overwhelming at this point to pick one specific system seller

GTA 5

Zelda botw was a game changer. It’s literally open world. After great plateau you have the freedom to go any direction you want. Some games give you options but in Botw you can do whatever. Revolutionized open world gaming.

BotW has been moving hardware since the console's launch, whereas Odyssey only has two months. Even if both games have the same selling pull, BotW has had way more time to do so, thus selling more consoles.