For the first time ever, Nintendo decided to launch two versions of Super Smash Bros. this generation, and the move appears to be paying off. 3DS has a much larger install base than Wii U, so having a portable version of Smash has increased sales potential greatly. According to Nintendo's latest investor report, Super Smash Bros. for Nintendo 3DS has sold 6.19 million units, while Super Smash Bros. for Wii U has sold 3.39 million.
This puts the total sales of the newest iteration of Smash at 9.58 million. By comparison, the original Smash sold about 5.5 million, Melee sold around 7 million, and Brawl, sold over 12.5 million. At this rate, this could very well be the most successful Smash generation to date.