Actually, Felghana's OST is the one I enjoy the less out of the three - I *like* it, but... I don't love it. It has five or six pretty awesome arrangements (Steeling the Will to Fight, Valestein Castle, Chop!!, etc) and then the rest are just... relatively good, I suppose. I personally prefer other versions (PC-88/98, X68000, Mega Drive, PC Engine CD, *insert Falcom arrange album here*) more.
I actually really love Ishibashi's more electronic and ambient sound of Ys VI's OST - tracks like Ultramarine Deep, Reconciled People, Windslash Steps, Mountain Zone, Defend! And Escape!, The Ruined City 'Kishgal' and The Depth Napishtim are some of my favourite tracks in the game and always put a smile on my face. And then Ys Origin got all those super sick Takeshita tunes such as Scarlet Tempest, Scars of the Diving Wing and Beyond the Beginning.