I do like Those Who Fight, but definitely Terra's Theme. As I mentioned in one of the other threads, it might be my favorite song in the whole series. I love how there's a sort of melancholy to it as well as a triumphant/adventurous feel - it's an interesting combination and perfect for Terra as a character. I also really prefer the in-game version to other arrangements that I've heard (orchestral, piano collections) since they don't, IMO, strike the same balance of emotions as well as the OST version does.
Terra's Theme! Not really a contest for me. There is so much more depth in her theme than there is in Those Who Fight. I do enjoy VII's battle theme, but that's all it is, a battle theme.
I like what Ghost and Mogget said about her theme, very much agree with it. There's several different emotions packed into this theme. Starts off like a melancholic adventure, then it seems to get a bit more hopeful (in a very bittersweet way), then boosts into that triumph. It's a tough journey and maybe a long one, but in the end, there will be something good, even if it's sad in other ways.