Song of Storms, defiantly. It's so trippy and wonderful. I really liked the Serenade of Water too, actually. I guess I have a thing for water-based songs in OoT :P
My favorite OoT song list:
Gerudo Valley theme
Serenade of Water
Song of Storms
Twinrova theme
Zelda's lullaby
Epona's song
They're in no order, they all give me extreme amounts of nostalgia xD