So far I'd say the most impressive Switch games visually are FAST RMX for its fancy rendering tech at 60fps, Doom for cramming a PS4/Xbone/PC game onto a mobile chipset while retaining most of its 8th gen graphics techniques, Mario Odyssey for great lighting and overall visual appeal at 60fps, and Mario + Rabbds Kingdom Battle for its lighting, effects, and AA.
Considering these all released within the first 9 months of the system's life, it's safe to say the best is yet to come for the Switch, and future games should push the graphical envelope further.