1/ WHAT IS A RIVAL IN ANIME : 

A rival is an antagonist, or anti-hero, he is simply the character who rivals the main character of an anime, mostly in power and character personality. Most common examples are: Vegeta from   dragon ball  and Sasuke from  naruto. 

    2/ WHY VEGETA IS THE PERFECT RIVAL:
  A) Character :
Vegeta is one of most popular "DRAGON BALL" characters, he beginned in the sayian saga from DBZ. Vegeta is the prince of the sayians, the mighty warrior race. He is and elite sayian and one of the strongest characters in dragon ball.
  B) Development :
Vegeta underwent what is called the positive character arc, where a character starts off evil and then turns good. He went from being a main villain who wanted to destroy the earth, a guy who used to ignore people like goku, a guy who killed his partner nappa, to an antagonist in the namek arc, he even teamed up with the heroes to fulfill his goals. Then , he ended up being a hero who defends the earth with his rival and friend Goku, he also has a family and people whom he cares about a lot, and he also acknowledges that GOKU, his rival, is better fighter than him. Plus, he also had big power boosts, now, he still rivals goku's power, he also rivals the power of the Gods in Dragon ball.

So, this is why i think Vegeta is the perfect Anime Rival.