VII and IV. In my opinion IV was the first real epic FF with in-depth characters and story. VI had a great plot too but to be honest I found it just a little bit too long-winded.
Well, if I must...
Music: probably VIII. Who can't love Liberi Fatali?
Plot: VII. The whole plot twist with Cloud did it for me. And Tif'a my favorite character ever.
Gameplay (battles and stuff): ......(very embarrassed)....X-2. Yes I know! But I actually found it quite addictive. Ahem, back to REAL FF's, I'd say X or III.
Characters: My favorite cast would have to be VII. I liked every character. Even Cait Sith.

So overall, VII.