I think Ippo and Sendoh are the perfect shounen sports rivals. (Certainly, they're my favourite rival pair.) The strength they see in each other excites them and they can't wait to step in the ring together and test themselves against each other. Sendoh, especially, is obsessed with Ippo. He gets so worked up just watching a video of one of Ippo's bouts that he jumps on a train from Osaka to Tokyo, on a whim, just to meet Ippo. (And while he's there, they run together along the embankment. ♥)
When they fight, it's with that pure sportsman's joy. And once is not enough, for either of them. Ippo's coach tells Ippo that he is the one that made Sendoh into an amazing fighter, because everything Sendoh did to become stronger was focused on fighting Ippo.
I can't tell you how much that makes me smile.
Which brings me to Kaidoh and Momo (and not just because both pairs are voiced by Kiyasu and Onosaka). I love those boys with a surpassing love, but for a different reason. Their story is not a rival story, at least not in the "present day" of Prince of Tennis. We get background about them trying to outdo each other, but we don't really see that in the current canon. They snipe and bicker and push at each other, but they don't work on their tennis with the goal of defeating each other. In the manga, we don't even see them play a singles match.
Instead, they play doubles. They learn to work together. I think that makes me smile even more.
Also, it's easier to slash them.