Now that the numbers are there time for some comparisons:
Mario Kart Series:
1. Mario Kart Wii - 37.24M
2. Mario Kart DS - 23.60M
3. Mario Kart 8 Delxue - 22.96M
4. Mario Kart 7 - 18.68M
MK8D is now in the league of the big 3, leaving all other games far behind.
If you add the original MK8 (WiiU) total then MK8 sits at 31.4M (22.96M + 8.44M)
So 6 millions to go to beat the Wii game. Soloing the Wii game is probably impossible.
Beating the DS game is only a matter of time. Probably happens in this current quarter (Q4/2019)
Smash Ultimate does not even need a comparison. It already sold Smash WiiU + 3DS combined and is currently just increasing its lead.
Super Mario Series:
3. New Super Mario Bros Wii - 30.28M
4. Super Mario World - 20.61M
5. Super Mario Land - 18.14M
6. Super Mario Bros. 3 - 17.28M
7. Super Mario Odyssey - 16.59M
8. New Super Mario Bros. 2 - 13.32M
The 3D games are no challenge for Odysseys sales. So comparing them against the whole Super Mario lineup it is currently at 7th and still has room to climb.
Hitting 20M and beating SMWorld is possible but NSMBWii will not be reached. But that makes it by far the best selling 3D Mario.
The Legend of Zelda: Breath of the Wild also has absolutely no competition in its franchise. (16.34M vs 7.6M of 2nd place OoT) Only an amazing sequel could come close.
1. Red/Green/Blue - 31.38M
2. Gold/Silver - 23.1M
3. Diamond/Pearl - 17.67M
4. X/Y - 16.44M
5. Ruby/Sapphire - 16.22M
6. Sun/Moon - 16.18M
7. Sword/Shield - 16.06M
8. Black/White - 15.64
Sword/Shield has a shot at becoming the 3rd bestselling Pokemon game in the franchise. Gen1/2 are almost untouchable but everything else is beatable. Having DLC expansion packs is also a first which could work wonders for the games legs and maybe carry it even further.
11. HeartGold/Soulsilver - 12.72M
12. FireRed/LeafGreen - 12.00M
13. Pikachu/Eevee - 11.76M
14. UltraSun/UltraMoon - 8.7M
Lets'Go has almost no legs at this point but it only needs one more Million to beat two games of the franchise. But the distance to UltraSunMoon is already big.