Seraphor
It may seem ridiculous that 'best selling games' = 'best games', because we all know quality isn't based on popularity. However it is the only objective way to rate them.
Any other reasons you give for any games being 'best' or 'worst' would be purely subjective and reliant on your opinion.
If we're just going by opinion then you'll be bickering perpetually because you have your opinion and they have theirs, unless of course it becomes a
popular opinion, and then it'll be based on
popularity anyway.
Sales figures are the only hard facts here, and lest we all forget the reason producers release these games in the first place, a good selling game is a
successful game, and thus can be claimed as 'best'.
On the contrary, there are several different ways that "best" might be quantified, even using numbers alone, and there is no single factor (either derived fact or popular opinion) can be used to arrive at a proper and absolute conclusion.
The folly of numbers is just as shaky as popular opinion. FFVII & FFVII International have a combined number of copies sold of 9.8m. So what did each individual version sell? FFX sold 6.6m between it's regular and International editions, but FFXII sold 5.2m of it's regular version alone (I couldn't find ZJS International sales figures). How many regular FFVII copies sold? By region? How about FFX? (Complicated some by the fact that the European release is also the International edition.) How about FFXII? What about the marketing money that was invested in each game? Or the development money? What was S-E's return on investment per individual dollar for each of these games? It's possible that even success can be measured by factors different than just copies sold. You can mix, match, and mince numbers just as easily as you can argue popular opinion to serve your own ends. Numbers are not the absolute hard facts you would make them out to be.
The raw power of sales figures are all well and good, except the numbers alone don't do a very good job of putting things in context. The original Star Wars netted a little under $7m in it's opening weekend. In comparison, The Phantom Menace managed a little over $64m on it's opening weekend, nearly ten times as much. Does this mean The Phantom Menace is a better Star Wars than Star Wars was? I have a hard time believing that most fans of that series would believe that, regardless of what the numbers say. Those numbers also fail to address several other factors without the proper context, but at face value they mean about the same thing as the numbers spouted about the FF games earlier. You might disagree with my numbers about Star Wars, so how would you combat this? You would get new numbers about it that serve your own opinion.
Sales figures are just a small factor in a larger issue, but looking to them as the sole factor to determine a game's greatness is foolishness. The honest truth is that there's no easy or clear answer to this question right now. With the passage of time, long after the FF franchaise has been left in the dust, when I'm a great grandma and if I still care about some old video game, perhaps the distance from this currently hectic battlefield will grant us the clarity to see what the best was in a more definitive fashion. That or I'll just use my walker to beat dissenting people into agreeing with me.