In my opinion 2017 by far, you had a revolutionary mainline Zelda game that's arguably one of the greatest games of all time and same goes for Mario Odyssey, it's already hard to beat a year where 2 of one of the greatest games ever release the same year. But it wasn't only Mario Odyssey and Zelda BOTW. 2017 Switch had Huge games releasing at a rate of once a month which is rare to see in any console. March-Zelda BOTW, April-Mario Kart 8 DX, June-Arms, July-Splatoon 2, August-Mario & Rabbids, September-Pokken Tournament, October- Mario Odyssey AND Fire emblem heros, November- Doom, December-Xenoblade 2.
I know that some of these are ports from the Wii U, but the majority of people who own a Switch never owned a Wii U which makes them seem like new games for people never being able to experience them. It's not only that we got two crazy games, we got a bunch of other solid releases that supplemented those games and made it one of the best Nintendo years ever.
While 2019 was a pretty solid year of releases for the Switch, I wouldn't really call any of the games close to BOTW or Odyssey level. 2019 did get a good amount of releases, but i would consider the releases in 2019 the B Team of Nintendo releases, not really Top tier Team A releases