Gotta admit N. Let's face it, he's one of the most interesting characters in pokemon. His purpose, his ideals, his backstory, his beliefs, they made him so three dimensional I'm surprised he doesn't have a 3d model. Jokes aside, the reason I consider him a rival is because from you first battle with him, I sure he could tell that you were the second hero he'd have to fight. So in order to show who's ideal's stronger, he constantly trains his pokemon to point where you reach the climax where he'll see who's strongest.
I would've gone for Hugh, but to me he sort of had a one track mind and was always angry, for an understandable reason, but never exactly treated you like a rival or friend till the near end. So he didn't have that "I need to be stronger than you" vibe.