Honestly, Final Fantasy IX is my personal favorite. It isn't as popular as I wish it was, but I can understand why, having it released at the end of the PS1's era.
This game is near perfection for me. Gameplay-wise it still holds all the standard Final Fantasy tropes, but the changes to it like the simple Ability system and even just having a party of 4 characters helped things along. The Chocobo Hot & Cold minigame is by far the most fun and rewarding sidequest line I've played, and even the card game, though stale, still gave a little bit of variety (btw, online Tetra Master? Please? Thank you)
The music? Out of this freaking world. Uematsu completely outdid himself on every track and every senario. It remains to this day the only video game OST I actually shelled out to have physical copies of.
Seriously, see what you've been missing.
Graphically I'll admit it isn't the greatest. It is a huge improvement over FF7 and FF8, but still uses prerendered graphics for most scenes. Thankfully the sprites do blend easier, and animations are suprisingly smooth. The tone has been described as 'bright and kid friendly', but I feel that is just the whiplash from the 2 previous dark and moody games (and anyone who says Memoria or the Iifa Tree is cartony has obviously never gotten that far)
Where this game stands out though is the story. The world FF9 creates is heavily reminiscent of your standard fantasy worlds, and has call backs to older Final Fantasties. It is very open and magical, but still has grounds in reality, even if it is sometimes cruel and miserable. And that is the main theme of Final Fantasy IX and why it is so powerful. Final Fantasy 7 focused on the impact someone has after death, and 8 is about dealing with tragidies, 9 takes the whole cake and literally focuses on the circle of life and death itself, and what life and the memories taken from life can mean.
tl;dr, FF9 is jut plain amazing, at least to me. There must be some reason the Final Fantasy creator Hironobu Sakaguchi said it was the closest to his original idea, right?