I didn't say final boss because even if you don't consider Yu Yevon the true final boss of FFX due to being impossible to lose against unless you turn yourself to stone, Necron is the final boss of FFIX and not Kuja. Not that his song isn't awesome like the rest of these are.
Otherworld easily. Love it so much that that's Kuja's ringtone in my Dissidia AU, even though otherwise he would have his Dark Messenger theme (and everyone questions why the hell does he have Jecht's song as his ringtone, but it's FUCKING METAL [Monster Prom reference, hehe], that's why).
