Pokemon Sun and Moon will most end up in the top 2-3 best-selling Pokemon games of all time after Pokemon GO reinvigorated the franchise for a lot of gamers.
In just over two months, Pokemon Sun and Moon has already outsold Pokemon Omega Ruby/Alpha Saphire and Pokemon Black/White 2. It will likely outsell Pokemon X and Y and Pokemon Black/White in the next few weeks.
Unfortunately, Pokemon Red/Blue/Green and Pokemon Gold/Silver are likely not to be reached but this is due to the fact that this was prime Pokemon time. When compared to releases in the last 10 years, Pokemon Sun and Moon is tracking to be well ahead.