Correction: Final Fantasy XI has been the most financially successful game for Square Enix. Ever. It singlehandedly brought them from the edge of the fiscal cliff they faced after the colossal failure that was Final Fantasy: Spirits Within.

That said, Final Fantasy XI blows most of the other titles in the series out of the water in the sheer depth of the world and story. Only Final Fantasy XII equals it on the level of lore depth, and even then mostly only through the beastiary entries. The only thing I can say that's disappointing about FFXI on a story level is that your character is back to being the voiceless protagonist who really never influences anything outside of battle. That said, the cast of supporting characters is absolutely phenomenal.