Each of them Had better Aspects imo, not one dominated everything... all of the FF's were memorable for me except FF3, I've played 3-13. for me My favorite is X. I list top 3 in each category for me
Story
FF10>FF6>FF7 Crisis Core
Characters
FF10>FF7>FF5
Gameplay (battlesystem)
FF13>FFX-2>FF12
Gameplay (Management System, Job System, Junction System Etc)
FF5>FFX-2>FF8
Minigames (not really close for me on this one)
Triple Triad (FF8)>>>>Sphere Break(FFX-2)>Fort Condor(FF7)
SideQuests
FF7(Yuffie Chocbo A lot of others)>FFX-2 (though it's arguable what is and is not a sidequest so this might not fit)>FFX
Music
FF8>FF6>FF7
Artstyle(Not character)
never see that in the game besides character portraits (character portraits do count though))
FF13>FF10>FF9
Artstyle(Characters)
FF10>FF5>FF9
World/Lore This part beneifts a lot from that these games have direct sequals
This, I will do a tie... because I can't decide (Sorry Cop-Out)
FF7,FF10,FF12,FF13