in terms of:-

story line

gameplay

characters

etc.

for me it has to be FF 10, till today i feel its the most engaging in the series.

in terms of:-

story line: X and IX

gameplay: probably VI with all the varied and weird moves xD

characters IX, X even though IV,V,VI al had nice casts aswell :P ( I like the happy guys more xD)

Music : IX and X

Baddy : VII, X and IX

stat, level etc System: VIII and V

etc.

Overal I like IX the most, closely followed by X.

Story -- FF6, FF7, FF8

Gameplay/Battle system -- FF13, FF10, FF7

Characters -- FF6, FF13, FF7

Music -- FF6, FF9, FF8

World/Setting -- FF13, FF6, FF12

3 most memorable for me in each section.

I think FFV is the most memorable for me.

Story- IV,VII,X

Gameplay-IV, V, XII, XIII

Character- IV, VII, X

Music-X, XII, XIII

Crisis Core and FF12

Kingdom Hearts in everything.

(Yeah, I cheated, so what?  Wanna fight about it? )

Can you guys just choose one game please?

Final Fantasy X had it all.