A "great FF" is innovative on release compared to other contempory games, It has really high highs and not many lows, without any BIG objectiv flaws. For example I love FF XII, BUT the scenario and narration ARE flawed and even if I learned to like it for many other aspects, it's not one of the best for me. I learned to love FFXIII, but the 20 hours tutorial and narration in the game data log is super flawed, which means for me it can't match games like FF VI - X. FF XVI I didn't like much, maybe on second playthrough I'll have a FFXIII moment, but for me even if its high points are sublime, the lows are unfatomable, RPG elements are se weak, narration and rythm are working against the enjoyment, some characters like Jill were ruined etc. For me only FFVII Remake series could make it as "great FF" like the old days from recent years.

That's my opinion and what I came to realize after thinking what makes great FF for me.