Easily FF6. I can tell you most things about that game from memory, which I can't really do with any other FF. I can do a fair bit of 4 that way, and some of 1, but... that's about it as far as FF is concerned.

Overall

FFVI, IV, VII, I, and IX are very memorable to me

Final Fantasy I Since it was my first and really liked the battle system.

Final Fantasy IV for the battle system & story.

Final Fantasy VII for pretty much the total experience. I especially like the art style the game uses.

Final Fantasy IX because of the characters.

IV and VII

its kinda hard every FF is great in its own way but if i have to select one then it's FF X it was the most engaging one!

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

Final Fantasy 6 in pretty much everything outside of some music parts - which I liked better in V.

Everything about Final Fantasy VII Crisis Core.

Zack death.... noble sacrifice.. I can never ever forget that. I cried :(