Bit of a random question, but see title.
For me, it's Conquest's: Resolve (Dark). I quite like the transition into the Flute segment.
Links:
Enemy Phase (Dark Dragon and the Blade of Light)
Rebellion (Mystery of the Emblem)
Sally (Thracia 776)
Prepare to Charge (The Binding Blade)
Prepare to Charge (The Blazing Blade)
Combat Preparation (The Sacred Stones)
Raising Morale (Path of Radiance)
Backsliding (Radiant Dawn)
Pray for Victory (Radiant Dawn Endgame)
Preparations (Shadow Dragon)
Before Battle (New Mystery of the Emblem)
"We'd best prepare for combat, just to be safe" (Awakening)
"Fate won't change, after all." (Awakening)
Resolve (Light) (Fates: Birthright)
Resolve (Dark) (Fates: Conquest)
Those Who Visit (Fates: Revelation)
Preparations: Deliverance (Shadows of Valentia Alm Side)
Preparations: Pilgrimage (Shadows of Valentia Celica Side)
Preparations: Reunion (Shaows of Valentia Endgame)
I don't think FE2 and 4 have true prep themes, but if they do correct me and link them.