Just because a system sells the most doesn't mean it has the best games for everyone.
I believe that if one can get only one system then they should do some research and see which one offers the most games which appeal to them. In my case, despite the huge library of games for the PlayStation, there are far more games on the Nintendo 64 which I like. Same for PS2 vs. GameCube.
In the case of SNES vs. Genesis, if I went by your logic of sales = best, then it would depend on which region you live in. In Europe and Australia, the Genesis (known there as the Mega Drive) sold far better than the Super NES did, which would make it "THE" system to own.