OF NOTE: Nintendo's only running down its first-party games here.
It's entirely possible — but very unlikely — that a third-party game is the top-selling title on Nintendo's Switch. "Minecraft," for instance, has likely sold a lot of copies of Nintendo Switch. The same could be said for "Diablo 3" and "DOOM."
That said, these games are unlikely to have reached the sales numbers represented by the top five games on the list below.