whoa could you possibly post a tougher question?^^
Alrights lets reduce this to the absolute favs:
Final Fantasy VI
Terras Theme (orchestral and piano)
Battle Theme (Black Mages version - greatest battle theme EVER)
Decisive Battle (Black Mages version)
Final Fantasy VII/Advent Children
Those who fight (normal and piano version)
Those who fight further (Black Mages version)
Those chosen by the planet
Cids Theme (desperately want an orchestral and piano version of that one!)
J-E-N-O-V-A (orchestral)
Tifas Theme (piano version)
Aeriths Theme (orchestral and piano versions)
Opening - Bombing Mission (orchestral)
One Winged Angel (orchestral)
World Crisis
The Promised Lands (one of the most amazing pieces ever)
Battle in the forgotten City (normal version)
Divinity I and II (epic!!)
Advent: One Winged Angel ( absolute best boss battle theme of all time)
Cloud smiles
Final Fantasy VIII
Liberi Fatali (orchestral)
Force your way (Black Mages version)
The Landing (really want an orchestral version of this one)
Fithos Lusec Wecos Vinosec
Love grows (orchestral version)
Ride on
Maybe I´m a lion (Black Mages version, listen to this version while battling Ultimecia/Griever - holy motherf....)
The Extreme
Ending Theme (normal and piano version)
Eyes on me (all versions really^^)
Final Fantasy IX
Hunters Chance (Black Mages version)
Vamo ala Flamenco
Battle Theme
Loss of me
You´re not alone
Terra (really need an orchestral version of this one)
Dark Messenger
Melodies of Life (all versions)
Final Fantasy X
Otherworld (normal and Black Mages version)
Enemy Attack
Path of Purgatory
Revelation
Yunas Theme
Aurons Theme
Rikkus Theme
Song of Prayer
The Dance
Suteki da ne (normal and orchestral versions)
Ending Theme (normal and piano versions - most beautiful ending theme ever)
To Zanarkand (orchestral version, the most powerful and my most fav FF song of all time)
Final Fantasy X-2
Eternity
1000 Words (normal version)
Demise
Yunas Ballad (orchestral and piano versions)
Reunion
Under Bevelle
Kimi He (lyrical version of Yunas Ballad - SO beautiful)
Final Fantasy XI
Distant Worlds - Memoro della Strono (orchestral version)
Final Fantasy XII
Esper Battle
Theme of the Empire
Kiss me goodbye
Final Fantasy XIII
The Promise
Serahs Theme
Fabula Nova Crystallis
Fabula Nova Crystallis (orchestral - different version then normal blended with Serahs Theme)
Sunleth Waterscape (what an earcatcher!)
Blinded by Light (orchestral)
My Hands
There are also a couple of songs from the FF XIII-2 OST that I really love but I dont know the songs names so well. Really love the rocksong battle theme, Serahs Theme, Lightnings theme, Yeuls Theme, Caius battle themes and especially the one that plays when you control Serah for the first time in New Bodhum.