Vivi vs Donkey Kong

An enduring contest rivalry that has been contested across the decades and a part of the larger Nintendo vs Square rivalry. It's notable for a number of reasons.
 * It's never become oversaturated. Rivalries such as Mario vs Crono were an annual occurrence in the early days of the contests, but even ignoring the fact that contests themselves are no longer an annual occurrence, they've never faced each other in consecutive contests or within five years of a previous clash.
 * It's never been muddled by gimmicks. There are plenty of other pairs of characters that have clashed more often, but they've counted on things like fourways and Rivalry Rumble to pad their numbers. (Heck, Squall and Sora have faced each other five times without ever having a clean 1v1 match--twice in each fourway contest and once in Rivalry Rumble.) Vivi and Donkey Kong have only ever met in 1v1 matches.
 * It's been remarkably consistent. Vivi's first-ever contest match was saw him get 56.47% on Donkey Kong. Six years later, they met again and DK was able to improve by all of .18%. Another eight years passed, and Donkey Kong appeared to be stronger than ever in his first two matches (the first time their rivalry wasn't contested in the first round)...and it was only enough to make it a 55-45 match.