Now, drawing on my own experience in the FF series, I rank Zidane/Garnet as the best love story because each of them had plots independent of that love story. It was not the immediate "zomg I love you!" of Aeris/Cloud or Tifa/Cloud or Yuna/Tidus. The fact is that both of them had to grow before they could love one another, venturing apart and discovering themselves. Rinoa only ever existed in FFVIII to be Squall's love and to be in that story. She had no other purpose until the horrible plot twist of her being a sorceress. Aeris spent most of her screentime falling over and flirting with Cloud and Tifa was the same, but kept it hidden.
Zidane's and Garnet's romance was perfection.