I like X. It has a really chilled and kinda depressing vibe to it that fit the atmosphere of the game perfectly. And it had a lot more composers than Uematsu so it was a collaboration of many different styles.
It's hard to choose. Overall, FFIX takes the cake for sure. However, if you break it down, it can seriously differ. For the sake of this question, I choose FFIX. However, FFIV has the best boss music, FFXIII has my favorite town music (Dust to Dust), FFIX has the best mini game music (Vamo Alla Flamenco), etc, etc.
I am the Way and the Truth and the Life; no man comes to the Father except by Me. -The Lord Jesus Christ. John 14 verse 6.