Super Smash Bros Ultimate (15.71M)
Best selling single SKU of the series.
Only 2.96M behind the combined total of Smash 4 (Wii U + 3DS)
Pretty much guaranteed to take the top spot by itself.
Your mathematics are a little off because it has already beaten Smash 4 combined sales which currently sits at 14.91m ( 9.54m 3ds and 5.37m wii u ). The official sales data is in Nintendo's financial report in the best selling software section.