Mufasa and Sarabi, definatly.
I think all the couples (in the movies) are wonderful and perfect in their own ways.
But for me, Mufasa and Sarabi are just the utimate, ideal couple. They just seem so equal.. Where the other couples (Simba/Nala and Kiara/Kovu) have one "weak" and one "strong" part. Where would Simba be without Nala to guide and support him? And need I mention the transformation Kiara made Kovu go through?
I think Sarabi and Mufasa are both strong and wise, they are ageless. They are the only established couple, the only one that has already fallen in love and been together for a long time. They have already found their places in the Circle of Life.
They are the ultimate King and Queen, perfection!
<3