Let's see, for me 2017 stands out for Zelda, Mario Kart, Splatoon, Mario+Rabbids, Axiom Verge, and Odyssey. That's an insane line up.
2019 for me had Tetris 99 which I like a lot, a bunch of old final fantasy's got ported over which is awesome. Mario Maker 2 is fantastic, Pokemon and NSMBU are pretty good, Link's Awakening I haven't gotten yet but really want to, Ring Fit which I just played today and I was super surprised by and might just get, they added SNES to the subscription service which is nice, and other games that look cool but I won't end up getting.
2019 was great, and I didn't even partake in several of the good games that I would probably enjoy that came out. But it's real hard to beat 2017's combo of Zelda, MK, Splatoon, M+R, and Odyssey (as well as Axiom Verge which was a new game to me and i loved it!).
2017 had better highs, but 2019 had more good stuff to get into. Overall I'd probably take 2017 just because I don't need more than a handful of incredible games in a year. But 2019 has done a much better job making Switch's game library more rounded and diverse.