All of her songs/PVs.
Every song & PV has their own meaning and symbolism, so it's really hard to say which is the most 'meaningful'. I have cried while listening to glitter.... so yeah.
But right now I'd say
Rainbow. (how come no one has said that yet?) I can't even describe it. Both the song & PV feel so simple but so 'high above' at the same time, you know... sad and happy.