My reason for loving IX, was that the game was a sort of allusion to the franchise itself.
Characters were essentially caricatures of various existing "jobs" in the series, yet were still relatively complex and full of development throughout the game.
Whether it was Zidane's initial misled womanizing guised as saving a damsel in distress, which later evolved into a matter of self discovery and subsequently, loathing, which with the support of his allies and love interest, overcame the lie he has lived all his life.
Or perhaps the curious existence of Vivi, who lives only in the moment until the strings of fate thrust him upon a war, using mindless dolls of destruction in his likeness, leading him to question his existence and purpose in life. Where did he come from? What is he living for? This is something that can echo and relate to just about anyone.
Steiner, though initially annoying and haughty, comes to question his initial sense of duty, and comes to terms with his obligations and the consequence they hold not only to guarding Princess Garnet, but to the world as well. Evolving from a prude bodyguard, to a truly noble guardian, whose cause and purpose is pure and sound.
Garnet, while on the road to discovering her history, and the answers to the nightmares that have haunted her for years, and witnessing the unspeakable horrors committed by her surrogate mother, swears unyielding fealty to her country, its people, and more importantly: her friends. Never compromising justice or compassion, she holds inner strength and will that is unparalleled to any character in the entire game.
And Freya, who despite losing losing her entire country and race of people to the genocide of Queen Brahne, along with the love of her life to amnesia, clings to hope. After facing a fate she herself would deem worse than death, she motivates herself forward, for the sake of those lost and loved, she is determined to make herself worthy of legend for the history books, than to be left behind and forgotten within the pages, where many others have since gone. She will be remembered, she will never, ever be forgotten.
I could go on, but I rather the characters speak for themselves.
And all of this has nothing to say to the sublime soundtrack, the effective and concise gameplay mechanics, the extensive yet rewarding mini games, extensively detailed environments, homage to the familiar and lovable elements that make a Final Fantasy, and overall warming experience to conclude this stellar package that rightfully deserves its place as one of the best titles in the franchise.
But alas, that's just my 2 gil. Different strokes for different folks; I won't argue that.