The Switch shares a wide range of Nintendo games with older systems, from Breath of the Wild to NES titles, but what do you think is the best first party game that can only be played on Switch, and what makes your pick worthy of the title?

Astral Chain. Best combat system ever. Fantasticly realized world with great level design, puzzles, soundtrack and awesome stylish visuals.

Metroid Dread is runner up.

Not sure if I would call it the "best", but I really love the "Famicom Detective Club" remakes. I played "The Girl who stands behind" some years after beating the SNES version, and it showed a lot of improvements.

That's exactly how a remake should be done.

For me, Smash.

for me, Smash. Smash is always up there every gen as my best game of the gen and Ultimate certainly didn't disappoint! Ultimate is so accurate. So much choice, so much fun! 1 of my fave games of all time tbh. 

Luigi’s Mansion 3 was everything I could’ve wanted! Narrowly beating out Breath of the Wild for my favorite Switch exclusive.

LM3

Astral Chain

As a wise man once said: "Best combat system ever. Fantasticly realized world with great level design, puzzles, soundtrack and awesome stylish visuals."

Not sure I could name one. You limited the field quite a bit.
Remind me what "first party" is again? Astral chain was made by Platinum who is third party company.
But Nintendo owns the IP. So is that first party? or Second?

So do you want Nintendo owned, Nintendo made, only on Switch?

edit: Also does similar to wii u count? Like Pikmin 3, or Mario kart?

Fire Emblem Three Houses