FFIX, some of it was very, very corny; the romance between Zidane and Garnet was arguably the cheesiest and least substantive in the series.
Can't see this, sorry.
Zidane and Garnet were the best FF romance. Their characters weren't sacrificed for the relationship and they both grew as the game progressed. Only when they had matured did they finally admit their feelings.
As it should be.

The least substantive was easily Squall and Rinoa.

Not taking sequels into consideration, I'd say FFIX. It was just such a wonderfully amusing game.
Guado genocide anyone?

Though it's still pretty dark if you look up suff about the Crimson Squad. Like how every single person went insane and killed each other save for Paine and co..

The Den of Woe was an awesome area.