But Final Fantasy 7 was the first Final Fantasy games (and conceivably one of the first RPG games) to make the translation into 3D.
That's probably a major part of why it was so widely praised. Enix probably knew that it would have to take the plunge into 3D at some point, and making an (albeit relatively crude) 3D Final Fantasy gave Enix the experience to improve on later.
Exactly! Same thing with SM64, and the like. The game that changes the series, and makes it into the current iteration, is most likely seen as the 'dominant' one, since it created all the traits now associated with the series. Super Mario Bros. 3 is a fantastic example, and so is Super Mario 64... 3 introduced many of the series, now, staples, and SM64 introduced the 3D gameplay. Sunshine and 2 are not as well received, because they have gameplay ideas that are not seen in the newer games.
Another example? Link to the Past, and Ocarina, were both games that heavily changed the series for years to come, and revolutionized the mechanics. Adventure of Link and Majora's Mask used gameplay ideas that were not kept.
What this also, as a result, means that a game obviously becomes more received over time, but also over sequels. The more common the trait of a game becomes over generations, the more accepted. Any 'outlier' in the scale of relations to other games is not as well received. Maybe if the game had stayed 2D, Adventure of Link would be the pinnacle of Zelda. Or, if the game had kept Mario 2's ideas, that would be the pinnacle.
This also means that even if a game is well received, it still needs more sequels keeping the mechanics to become truly classic.
And, if a game radically and rapidly changes the existing common staples of the series, it is also seen as 'not as good'. Sunshine is a glaring example.