Moria was definitely not easily beaten, Luffy had to attack him in all of his toughest modes (that we've seen so far) and was completely knocked out asleep by the time Kuma came into the picture. So yeah, I would definitely say that Moria was way up there in the list of toughest fights.
But I don't really buy this "rival" thing. The main difference with Moria and Luffy is that Moria doesn't like to work. His whole point in gathering tough zombies and underlings was so that they would take care of everything for him. Remember how lazy he was? It was also quite clear that he lost a crew to him at some point, and having zombies took away that fear of loss. He also joined up with the WG and allowed himself to be named as one of the Shichibukai. IMO, that already disqualifies Moria as a "rival" of Luffy's regardless of his strength or power or ultimate intent to be Pirate King.
If anything, the only character that fulfills that role is Shanks, because Luffy does everything to fulfill his promise to reach the top and surpass Shanks. It will also be interesting to see how the Strawhats interact with the recent Supernovas, since being newcomers, they haven't had any of their dreams shattered yet and are probably all still aiming for the top, just like Luffy is.
I will agree that I am interested to see what role Moria, Absalom, and Hogback will play in the future, seeing as how he made a point of showing us that they got away and are sailing to some new destination.