I agree that Penn-GSP 2 was a superfight, and I think any champ vs champ bout should be viewed as a superfight, regardless of number of title defenses from either fighter. I actually prefer there to be as few defenses as possible, because the more title defenses a fighter has, the least likely he is to challenge/accept a challenge from a champ from a different weight class.
So, if two one-hit-wonder champs fight it's a superfight? GTFO