It's really hard to narrow it down to just one, but the first game that comes to mind actually is Smash Ultimate.
So beautiful, especially coming from how underwhelming Smash 4 looked compared to other Wii U titles. Ultimate maintains the 1080p and steady 60fps of its predecessor, but this time delivers big time on the eye candy. All stages are gorgeous, the characters look great due to the new textures and animations (most of them still use the same models), the menus/HUD look super clean, as do all of the new animations and effects for just about everything that happens in battle. It's really, really impressed me.