The Great North Cave is deliciously ominous. It plays when you travel the map with the Highwind and it illustrates that Meteor is ahead and brings death and destruction. Even when I wanted to lose time in side quests (Chocobo breeding!), this music was always there to remind me what was truly happening.
Who Am I? is a perversion, you could say, of the theme of FFVII, which could be called Cloud's theme as well and it fits so much Cloud's question that haunted him during the majority of the game. It shatters his mind, so his theme gets twisted too.
Weapon Raid is PANIC, DESTRUCTION, RUN!!!
Highwind Takes to the Skies is the breather we need. I always picture the airship flying in the sky, with the group reassembled, determined and going to kick ass. After a lot of doubt, failures and desperation, they face the future with guts and confidence: Cloud's back, there ain't offa this train we're on, let's mosey!
Off the Edge of Despair always brings me to mind the picture of Cloud as a puppet, walking with strings to the rhythm of this piece forced by an unseen force. (I have a lot of visuals, leave me alone!) It's creepy as hell to my ears.
On the Other Side of the Mountain is... nostalgia and casualty to me. After the tragedy and epicness, another breather, only of different nature. It gives the impression of normalcy, routine and while alone isn't much of an impressive piece, it works. It shows a time when things were simpler, more innocent, even.
THIS IS DIFFICULT, MAN.