Well I'm going for Zorn and Thorn - as much as I don't like the characters, the theme tune is somewhat clever with the way the high and low parts of the main piano piece dance around each other hand-in-hand like the two jesters

Otherwise I might have picked Eiko's, but it's opening almost makes me think it's going to break into Yuffie's theme instead The two of them might actually be interesting played on top of each other, come to think of it