The Legend of Zelda: Tears of the Kingdom — 395 hours.
The Legend of Zelda: Breath of the Wild — 270 hours.
Super Mario Odyssey — 235 hours.
MarioKart 8 Deluxe — 190 hours.

Animal Crossing by miles. Why? Because I 100% museums when I play them and Nintendo keeps making that a lot harder with each game lol

Mine's Monster Hunter Rise at 150 hours, mostly cos of how thrilling it is to slay monsters ten times your size in weighty epic combat in order to craft cool new gear.

Next up's TOTK at about 80 hours; great game thanks to its emergent mechanics.

Animal Crossing: New Horizon. Over 750 hours. Played it pretty much every day for over a year. Carrot on a stick - the game.

Splatoon 3 : 2005 hours
Tears of the Kingdom: 285 hours
Breath of the Wild: 275 hours

As far as Splatoon 3 is concerned I believe some of that playtime is also on the Switch 2. It is also my most played game on that system.

As to the why ... I thought both Zelda's were fantastic and unlike most I think Tears is better than Breath. All the new caves, the Depths (which I really enjoyed exploring) and basically everything the first game had to offer. As for the Depths I just loved shooting an arrow with a brightbloom seed attached and not knowing if I was going to hit a wall or watch it arc down into a chasm way the hell away. It felt like true exploration and while it was not most people's cup of tea I REALLY enjoyed it.

I have been hooked since Splatoon 1, I have always enjoyed shooters and this take with it unique "reloading" and movement anywhere your ink is (including up walls) focuses just as much if not more so on movement / platforming and strategy as it does on your aim. While it has its flaws I find it to be a ton of fun and the different game modes on offer put objective and teamwork over kill count if you want to snag a win.

Fire Emblem Three House. I have an embarrassingly high number of hours in that game. At work so I can't see the exact number but I know it's over 1000 hours. 

1. Taiko no Tatsujin: Drum and Fun (200 hours)
2. The Legend of Zelda: Breath of the Wild (115 hours)
3. Xenoblade Chronicles Definitive Edition (100 hours)
4. Nintendo Entertainment System NSO (95 hours)
5. Xenoblade Chronicles 2 (85 hours)
6. Xenoblade Chronicles 3 (80 hours)
7. Hollow Knight (75 hours)
8. GameBoy Advance NSO (70 hours)
9. Mario Kart 8 Deluxe (70 hours)
10. NEO: The World Ends With You (65 hours)

Wow… that is a lot of hours put into Splatoon 3. I haven’t even put 2000hr into all my gaming combined since September 2022. How do you do it? I would love to put that time into a single game!

245 hours on Botw, really enjoyed the game and played the master mode and DLC. Took my time to explore
210 hours on Diablo II... i have just always been in love with this game and could play it forever. So glad it was ported to Switch.
Going on 200 hours on Totk, i still haven't finished the game. I feel like it's better than Botw for what it brings to the table. But i feel like i'm dragging myself a bit exploring each corners of the map again since i've already done it, but have to to complete the caves and wells. So it's like a love still being in this era of Zelda, but still kinda getting excited on finishing the game for good.

I got my brother-in-law into it, then my wife.  We normally play Clam Blitz together anytime it is on and the wife and I are both available (so 2 hour blocks).   Spent a good amount of many weekends with a couple people from here playing the periodic Splatfests too.

It is also about the only game i play these days on any system.  I put about 55 hours into DK Bananza and 30-ish hours into Prime 4.  If I have 10+ minutes and just want to game a bit Splatoon is my go to.