I'm going with Zidane/Garnet. It was one of the best pulled off love stories in FF, second only to VII's IMO.(I like how VII's took a cliche romance[her/childhood friend] and pulled it off in a non-cliche way)
I always go back and forward which pairing I like better but I always go to Garnet and Zidane. Partly because despite them both being good pairings, Garnet and Zidane are charming characters separately as well- and it just adds to how much I love them. I love every minute with these characters.
It's a hard choice for me because FFX was the first of the series I played and the story made such a grand impression on me. I just started my first playthrough of ffix a couple of weeks ago, and the romance story is so very real unlike the other FF relationships and is a classic to me already.
I'll vote Zidane/Garnet and leave my biases aside, though Yuna/Tidus will always have a corner in my heart as one of the greatest romances in a game.