I like all 3 of the Ballads from 7-9 (Aeris's Theme, Eyes on Me, Melodies of Life), as well as the song that Celes sings in the Opera House in FF6 and OWA. But my favorite would be Melodies of Life. I downloaded the piano music by Arnold Morrison, and also the one by Michael Huang to it. I play it 8 or 9 times a day. Not only is the music gorgeous, the lyrics I love and they actually RHYME!
For those who don't know the lyrics, I'll randomly post them:
Alone for a while, I've been searching through the dark
For traces of the love you left inside my lonely heart.
To weave by picking up the pieces that remain,
Melodies life, love's lost refrain.

Our paths, they did cross, though I cannot say just why.
We met, we laughed, we held on fast, and then we said goodbye.
And who will hear the echoes, of stories never told?
Let them ring, out loud, till they unfold.

In my dearest memories, I see you reaching out to me.
Though you're gone I still believe, that you can call out my name.

A voice from the past, joining yours and mine,
Adding up the layers of harmony.
And so, it goes on and on, Melodies of Life,
To the sky beyond the flying birds,
Forever and beyond.

So far and away, see the birds as they fly by.
Gliding in the shadows of the clouds up in the sky.
I've laid my memories and dreams upon those wings.
Leave them now, and see, what tomorrow brings.

In your dearest memories, do you remember loving me?
Was it fate that held us close, and now you just leave me behind?

If I could leave, this lonely world behind,
your voice would still remember our melody.
And so it goes on and on
Melodies of life, come circle round and grow deep in our hearts,
As long as we remember.