Cannot be decided.
Story wise: Crisis Core: Final Fantasy VII
Gameplay: Final Fantasy I, Final Fantasy VII, Final Fantasy XII, Final Fantasy XIII, Dissidia: Final Fantasy, Crisis Core: Final Fantasy VII, Final Fantasy III.
Optional stuff: Final Fantasy XII
Best characters: Final Fantasy VI, Final Fantasy VII compilation, Final Fantasy VIII, Final Fantasy IX, Final Fantasy X, Final Fantasy XII.
Best design (on enemies, spells, abilities, etc): Final Fantasy XII, Final fantasy I, Final Fantasy VI, Final Fantasy VII, Final Fantasy X, Final Fantasy XIII, Crisis Core: Final Fantasy VII.
Best music: All of them.
So in the end, Crisis Core or Final Fantasy XII? But FFXII don't have taht much of a great story, nor same great soundtrack as previoud games. Crisis Core is pretty repetetive when it comes to missions and is pretty decisive when it comes to higher stats.