I really wanted to give it to 2019, but in the end I voted 2017. 2019 had a ton of single player games, most of which I haven't touched yet. The only single player games I played in 2017 were Breath of the Wild and Mario + Rabbids, both of which I thoroughly enjoyed especially Zelda. But I also played a lot of Puyo + Puyo Tetris, and Mario Kart 8 D, and played some Arms too. 2019 didn't have any multiplayer experiences like that.
What 2019 has over 2017 is third party offerings, especially indies. I really Untitled Goose Game and Wargroove, and I'll throw Cadence of Hyrule into that category too. All 3 games are great and unique. I also just bought a bunch of indie games that I haven't touched yet, but will soon. Indies really hit their stride in 2019, and of the 3 games I mentioned, 1 is a Switch exclusive and the other 2 released on Switch before PS4.
I still have yet to actually try any Nintendo first party games from 2019. They look good, but they didn't make me want to immediately get them like BotW or Mario + Rabbids. And of course 2017 dominated in multi-player, so I have to give 2017 the win.