I think the first game’s soundtrack fits better overall when you actually listen to it in-game, in the intended contexts. But for listening independently of the game, I like X’s music much better.

Favorite from Surge Concerto is Class::CIEL_N_PROTECTA;. It’s a really, really good autidory stim!

It’s harder to choose my favorite Ar tonelico song. Maybe Hartes ciel, melenas walasye. Melody of Metafalica definitely has my favorite OST of the trilogy.