Pretty much all the Final Fantasy games have one boss theme that is repeatedly used in place of the general boss theme to separate the boss in tone from all others. This does not include themes that are only played once, or even final boss themes. No, these themes are recurring, making the individual boss fights stand out together.

In Final Fantasy IV, we have Battle with the Four Fiends, used during all the battles with said fiends.

In Final Fantasy V, we have two: Battle on the Big Bridge, used during almost every Gilgamesh battle, and Decisive Battle, used during the battles against Exdeath.

In Final Fantasy VI, there's Battle to the Death, playing during the fight against Ultima Weapon as well as several bosses in the final dungeon.

In Final Fantasy VII, JENOVA is played during the first and third Jenova battle as well as the battle against Hojo.

In Final Fantasy VIII, we have Premonition, played during pretty much every Sorceress battle, except Adel's.

In Final Fantasy IX, there's Sword of Doubt (or am I forgetting another theme?), played while you're being destroyed by Beatrix.

In Final Fantasy X, there's not really a specific theme like that, but the two that come closest are Challenge and Assault. Challenge plays during arguably two of the hardest bosses in the game, S. Flux and Yunalesca, while Assault first plays during the assault on Bevelle only to come back during the fights against Sin.

In Final Fantasy XII, we have Esper Battle, first and foremost. There's also Clash of Swords which replaces the general boss theme every now and then.

In Final Fantasy XIII, there are quite a lot. The one that really stands out is Desperate Struggle, which comes up during quite a few important battles as well as the Cie'th Stone Missions. Honorable mentions go to Eidolon Battle, Fighting Fate and Test of the L'Cie.

All these themes help the fights they're placed in to be more memorable. So, which does it best?