On a purely technical level probably Witcher 3 despite all the downgrades from the other versions. It's the sort of high-end port everyone thought was impossible on Switch, a vast open world game that already pushed the PS4 and XB1 at 30 FPS. Somehow it still looks like the Witcher 3 and runs basically as well as any other console version. Even Doom and Wolfenstein 2 aren't as impressive since they are linear shooters.
Dragon Quest 11 is the best overall port since you really have to put it side by side with the PS4 version to notice the downgrades and what downgrades there are don't diminish the fact that it is really the definitive edition.
For Switch exclusives it's much harder to narrow down. BOTW, Mario Odyssey, Mario Kart 8, all look amazing thanks to a combination of stellar tech and art direction. Luigi's Mansion 3 I guess would be best on a technical level simply because it's newer and therefore the team was better able to maximize the hardware, but I think art-wise I prefer many games over it.