Honestly I've always liked the romance between Cecil and Rosa (FF4) best. They're already together, they clearly care about each other and support each other, etc. Outside of the events of the game, they have an established, stable, and loving relationship. Probably the most mature and realistic relationship in any Final Fantasy, and really across a lot of media in general. They don't have drama, there's no "will they, won't they", just two humans enduring hardship with each other. It's beautiful and is exactly the right amount of romance IMHO.