Personally, I think the best looking Switch game has got to be Luigi's Mansion 3. Honestly am blown away. Some Pixar level quality animations IMO. I would totally watch a series based on Luigi's Mansion!

Yeah but... Hollow Knight might be the best looking game ever made (until silksong) haha It will certainly be the one that holds up in a decade or even two.

Technically though; it's getting really hard to go past Luigi's Mansion 3, it's so beautiful. Doom 2016 still gets an honourable mention due to the human sacrifice panic button obviously committed in order to get it running. 

Astral Chain is one of those cases where it makes very clever use of relatively simple techniques (such as cubemaps for reflections) to produce eye-catching visuals without the need for system-pushing tech.

Theres some tough ones here... Luigi's mansion 3 looks phenomenal. Links awakening looks phenomenal. Astral chain is huge and looks superb. Then theres games like ori and the blind forest... hallowknight.... breath of the wild, mariokart.... I just cant decide which one looks the best to me.

In the end, and in my opinion, Switch has a lot of good looking games.

I haven't actually played it myself, but from what I've seen Dragon Quest XI is a strong contender.

Seems your crystal ball was accurate, now that DF's analysis, and indeed the game itself, is out. :P

Personally I didn't think that the witcher 3 looked that great, that being said, after a few hours of playing the sheer scale of what was going on really started to sink in. It is truly a technical Marvel when it comes to scope and scale and I'm amazed they got it running on Switch at all. It would top a few other 'best' Switch categories for me but the graphics took an understandable hit. Though not enough to detract from the game in any major way. 

It's between Fire Emblem: Three Houses and Pokemon Sword & Shield.

Don't forget the mind-blowing beauty that is Ark Survival Evolved.