Well, Simple and Clean didn't really fit, neitehr did Hikari. Passion and Sanctuary really are the KH theme songs, though.
In you and I (Sora and Kairi), there's a new land (in their hearts for Roxas and Namine to live in)
Angels in Flight
A Sanctuary, my sanctuary, yeah
Where fears and lies melt away
Music Inside
What's left of me (When they become a heartless, the nobodies are what's left of them)
What's left of me now
I watch you fast asleep
All I fear means nothing (I guess she couldn't think of more words here because it's basically saying like in the chorus, when I watch you fast asleep all my fears and lies melt away)
You show me how to see that nothing is whole and nothing is broken (I personally like this line. My interpretation is that no one is really whole, with a body, soul and heart, and that no one is really broken, like Xemnas was searching for a heart to be "whole". Axel was a nobody, and yet he was still feeling some emotion, like crying when Roxas went into Sora.)