According to TICGN, Smash 4 is the highest selling game in the franchise.
There is a caveat to the news, though, 8.12 million copies sold were solely for the 3DS. That's out of the total 13.13 million, which means that the Wii U only had 5.01 million sales. Technically, if we're counting console-only sales, Smash 4 performed the worse out of the series.
You would think that Melee would be the second highest selling Smash game, and if you did, you would be wrong. It's third with 7.41 million copies sold. Brawl is second with 13.10 million in sales, and Smash 64 is last place with 5.5 million copies sold.
These numbers are a little skewed because the recycling of old games like Melee and Smash 64 isn't taken into effect. That won't amount to much of a difference, but it's something to consider.
Now that the information is released prepare to see Melee elitists come out of the woodwork to defend their game as being the best despite its sales.