Nintendo apparently use multiple engines, but the main one these days seems to be Lunchpack, which was used in the Splatoon games, Animal Crossing New Horizons, ARMS, Switch Sports, and LABO.
It seems to work well for their purposes, and since they know their own hardware better than anyone, it's probably one of the best optimized. It may not push the boundaries of what's possible, but it seems adept making games that look clean and run smoothly.

I think Insomniac is engine and the Decima Engine are worthy of praise. The games made on them look very good and the games run absolutely fine. Ratchet and Clank can even handle ray tracing at 60 FPS and the image quality in general is quite clean due to the temporal injection reconstruction.

Decima, Frostbite and Snowdrop.

That Battlefront 2015 looked that good and ran at 60fps on PS4 still blows my mind.

Nintendo engines are out of this world.

The engine that BotW and TotK run in must be really good.

Ah yes, Frostbite, I knew I was forgetting something! Totally agreed about Battlefront, both that one and it's sequel looked insanely good for 60fps games on last gen hardware.

Need for Speed 2015 and Battlefield 1 on PS4/XBO also really impressed me, as did Battlefield 3 way back on PS3/360.

It's a shame we only ever got one Frostbite game on the Switch, Plants vs Zombie Battle for Neighbourville, but it looked good and ran well for a PS4 port.