FFX. Because I'm a lazy bugger, I'll merely say that I agree with everything toad626 said.

I was a shameless FFVII fangirl for several years and I was a bit hesitant when I began playing FFX, but it blew me away and I can't say I regret playing it
So one could say FFX taught me to be more open-minded. Or one could alternatively say that I'm talking a load of bullsmurf.

FFVI has some truly beautiful scenes story-wise but it's been so long since I played the game and I never I actually completed it - only watched the ending on the internet. But I do love it.

...my post went a bit off-track, but oh well.