I'm not very knowledgeable on some of the rivalries, but I am gonna have to ship no, wait, shipping is normal here ship and vote Reynard × Zexion~ The two characters have a very easily identifiable kind of rivalry that is not unlike those of childhood friends when looked back after time. Calling each other names, 1-upping each other on abstract stuff that only they know what they're talking about, like, you know, the infinity+1 sword etc. Cecil also add a special facet to this rivalry that is not usually present in other cases because, well, childhood. No, I'm not saying R&Z are childish... are they? because then it's toy shipping oh damn mind :p
Mata Nui × Tromba would come a distant second, for reasons I exposed already back when we were discussing the character concept of Tromba. One thing is that I feel this would be a fairly "slow" rivalry in terms of what constitutes taking off the gloves, etc, if you get what I mean, so this one is safe to wait on and be given the onscreen time when you are sure you have material to work with it.
“Thou hast forgotten the face of thy Lord. Remember, mortal, and fear pet me.”