If I absolutely had to choose one for Switch I suppose I'd go with either Mario Odyssey as one of the first and best true exclusives, or Monster Hunter Rise as it's by far my most played game on the system, and one I'd been dreaming about since the 3DS days.
Successful systems tend to be harder to nail down as they tend to have more noteworthy games.
Bet with Liquidlaser: I say PS5 and Xbox Series will sell more than 56 million combined by the end of 2023. (And over 130 million lifetime)