1. Squall and Rinoa
2. Laguna and Raine
3. Selphie and Irvine
4. Zell and Xu (just say it.)
5. (calm) Seifer and Quistis

Let's face it. Square loves the opposites attract theory.
Squall=Closed, keeps to himself and quiet.
Rinoa=Loud, obnoxious (in a good way), and open.
The whole theory of Yin and Yang work beautifully in the FF couples. Even the other games have the same thing.