Super Mario Odyssey.

It was EXACTLY what Mario needed after things got stale from the end of the Wii/DS era and throughout the Wii U/3DS era. With the constant stream of NSMB titles and 3D Land/3D World, playing with the similar settings, themes, and tones based on the SMB3 and SMW, and just how frequent they were: 2010 - NSMB Wii, 2011 - 3D Land, 2012 - NSMB2 and NSMBU, 2013 - 3D World. That's 5 games in a 3 year span. Fans were pretty tired of Mario at that point and NSMB gameplay style. They wanted Mario to take a break and then come back with something fresh and different like how it was before.
After the NES and prior NSMB Wii and Galaxy 2, we only used to get one mainline Mario game per console, and it was something different and groundbreaking each time.
The SNES had Super Mario World which is widely considered as one of the greatest games of all time and arguably the greatest Mario game ever.
The N64 had Super Mario 64 which ushered in a brand new era in gaming and revolutionized 3D platformers.
The GameCube had Super Mario Sunshine which expanded upon Mario 64's platforming challengers while also experiment w/ the formula and franchise in unique ways.
And then the Wii had Super Mario Galaxy, an absolute masterpiece that made use of the Wii's motion gimmicks while also leaving an incredible feeling of magic and majesty.

That last point is VERY important, because while Galaxy 2 is also a fantastic game, it was still just more of the same that we got from Galaxy 1. It was retreading old ground. It was still magical and majestic, but on a lower level than the first game because we already knew what to expect.
And then we got the NSMB games and the 3D Land/World duology, which took the 2D formula and put it in 3D. But they were so common and safe that it was almost boring. Like "business as usual." The same old "Bowser kidnaps Peach, Mario go save. Platform for 8 worlds." In some instances, it WAS boring. (Looking at YOU, NSMB2!)

Fans wanted a break from the series for a bit and when it came back, it wanted something fresh. Something different, something ACTUALLY NEW. Not in name, but feel. They wanted that same sense of excitement, magic, and majesty that the series had been sorely missing since Galaxy 1 & 2. Something that you would get playing those old classics from the NES to N64.
And when the Switch was about to launch and we got those first trailers of Odyssey, THAT was the most excited that fans had been in a VERY long time because it looked like we were FINALLY going to get our wish! The original trailer in the Switch presentation and seeing Mario traveling to all these different places like an ACTUAL city, the gameplay looking like it was ditching the 3D Land/World/NSMB style and going back to 64/Sunshine/Galaxy. Then, E3 2017, seeing Cappy and the Capture mechanic, hearing "Jump Up, Superstar" for the first time!
And then the game came out, and it was EVERYTHING we all were hoping it would be and THEN some! An absolute masterpiece from start to finish and yet another title that lived up to the legacy established by many of its predecessors. Mario was back, baby! And by extension, so was Nintendo! After the dark days of the Wii U/3DS era, and the absolute pressure that was on them to make the Switch succeed right out of the gate, this game NEEDED to deliver. Breath of the Wild and Mario Kart 8 Deluxe definitely got the Switch off to the right start, but it was THIS game that firmly cemented the Switch's status as THE gaming thing to have. (Up until the PS5/Xbox Series launch, anyway)

TL:DR - Super Mario Odyssey, on top of being just an absolutely incredible game, is also the game I feel represents the Switch era the best as well as Nintendo bouncing back from its 8th Gen ditch, silencing ALL doubters, haters, and naysayers, and proving ONCE AND FOR ALL, that they are NOT the company you should count out. Because while they may get knocked down (sometimes, by their own doing), they always get back up, and when they do, they come back bigger, better, and stronger than ever.
And that concludes my essay as to why Super Mario Odyssey is the Switch's best first party exclusive.

Bayonetta 3 will shortly become a close second

Not sure what context is meant in regards for best but if in regards to as a game it's still BOTW imo as its customisable gameplay still easily holds up and yields new things today. If it's in regards to moving units it will be MK8D. 

My point wasn't that Switch has a weak lineup. It's library is very strong. And some of my top Switch game are multiplatform; Doom Eternal, Witcher 3, Ori and the Will of the Wisps, etc.

This thread just isn't about that. 

Animal Crossing New Horizons with runner ups Switch Sports and Ring Fit Adventure for me.

Although, I think looking at the Switch’s exclusives alone misses the fact that so many definitive experiences of certain games are found on Switch. It’s like ignoring Resident Evil 4 on Wii because it’s the third time the game came out, even though the Wii version is the definitive experience as the IR aiming makes the game feel SO much more satisfying than the other versions. Mario Kart 8 was a mediocre Mario Kart on Wii U, but was a killer app and arguably the greatest Mario Kart game of all time on Switch, thanks to the expanded feature array (mainly) provided by the hardware: namely the portability and awesome local multiplayer that can go up to 12 players, all with their own screens. Unless you’re into mods, most Steam games are also better on Switch, IMO thanks to TV and portable usability.

And it’s not just about the local multiplayer experience either. Me on a chair, Joycon in the left hand, joycon in the right hand, big TV in front of me, beer on the side, joint on the other side… can’t get better? How about Switch in portable mode while laying in a hammock, beer in a cooler at the side…and it’s the SAME console. There’s nothing quite like the Switch experience :)

Again though I'm not saying ports can't be worthwhile, some of my favourite Switch games are ports, that just isn't what this topic is about.

Disagree with this. Performance issues aside, as we cannot say for sure which PCs are running each game, I feel most of games designed for PCs plays much better with mouse and keyboard, from menus to gameplay. Real time strategy, simulation games, story driven games, all plays better on PC for me. Games that use the touch screen play very nice with mouse too. 

Even shooters plays better with mouse, the aiming is just many times faster and more accurate using mouse to aim.  

And the genres we play better with controller are not detrimental to PCs, because we can just plug them.