Atma weapon battle on the floating continent. Its like, "oh look, a boss. shouldnt be too hard." Then you walk up to it and the battle starts. YOu just know, instantly, your previous thoughts were totally wrong. The music is tense and powerful, hes a great hulking beast and his speech is damn sweet. Its at this point you realise you dont have a chance in hell of beating him. Then, he casts meteor or flare, and you cant help but laugh at the thought of how much levelling up your gonna have to do. Its just a great feeling.
Also, the regular boss music is probably the best of the whole FF series. Its powerful, it starts off all atmospheric and "Dude, its a boss, lets rock, baby!". Then, it gets faster and more intense, and you just cant help yelling out "YEAH! EAT. MY. FIRE. 3!! EAT TERRAS OWNAGE! FEEL THE BURN! YEAH!!!!".
I'm easily pleased.