Breath of the Wild + Mario Odyssey makes 2017 hard to top!

Basically nothing in 2019 came close to BotW and Odyssey. And those two games alone can last anyone most of a year. 2019 was a fantastic year but 2017 still has the two marquee games most associated with the Switch.

Both phenomenal years. For me personally, it's a tie.

In terms of new games I think it's a bit of a toss up. 2017 had a huge impact with Breath of the Wild and Super Mario Odyssey. The games from those years that I've purchased:

2017:
Breath of the Wild (w/ DLC)
SnipperClips
Arms
Mario Kart 8 Deluxe
Splatoon 2
Mario + Rabbids
Golf Story
Rocket League
Mario Odyssey
Xenoblade Chronicles 2
LA Noire
RE: Revelations
Lego City Undercover
I Am Setsuna
Yooka Laylee

2019:
My Friend Pedro
Yoshi's Crafted World
Untitled Goose Game
Luigi's Mansion 3
The Witcher 3
New Super Mario Bros U Deluxe
Pokemon Shield
Fire Emblem: Three Houses
Turok 1
Doom 3
Grandia Collection

However, I've purchased all the games from 2017 that I pretty much want. From 2019 I still want to get Astral Chain, Marvel Ultimate Alliance 3, Links Awakening, Dragon Quest XI, A Hat in Time, Ori and the Blind Forest, Dragon Quest Builders 2, Tales of Vesperia, and Ring Fit Adventure

IDK, still a toss up for me. I'm inclined to give it to 2017 though just because of how much I love Xenoblade and BotW.

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.

Zelda and Mario combo beat everything else.

2019. It had Astral Chain

2017. Two of my favorite games: Breath of the Wild and Super Mario Odyssey were released. Also, there was the release of Stardew Valley on Switch. I didn't play it until 2018, but it helps the argument. Not to mention it was the most downloaded title on the eShop for all of 2017. Add in Skyrim (which I'd never played for more than a half hour on my brother's PS3), Xenoblade Chronicles 2, and others. Mario Kart 8 Deluxe I bought begrudgingly, but I enjoyed the new features.
2019 had a lot of strengths, but several slow periods. Not to mention certain flaws of a platform become more apparent when it's been out for 2 years. 2019 was an improvement over 2018, though.
2020 could be the best year of the Switch yet. Animal Crossing: New Horizons will likely be amazing, and BOTW 2 may release. Not to mentioned the rumored Switch Pro or a potential Super Mario Odyssey 2.

2017 BotW , Spla2oon, and Odyssey were just such great games. Links Awakening is the only thing I bought myself for the Switch in 2019.

BOTW and Odyssey in the same year?? That's tough shoes to fill