I love Chrono Trigger but I find that Chrono Cross has a remarkable soundtrack that stands on its own, and I'm not talking about the Celtic Tracks.
For example, the main theme "Time's Scar" is great, as is its "Another World" arrangement. Then there's the rocking Magical Dreamers (arrangement of Marbule, played by rocker Nikki), as well as Fort Dragonia and Isle of the Dead, to name a few.
They're all pretty hummable and unique, but make up a great OST.