It's not uncommon for many anime protagonists to strive to be the best at something that seems horribly out of reach. Since the first episode of Pokémon, Ash Ketchum wanted to be the greatest Pokémon Master in the world, and throughout Dragon Ball, Goku has always wanted to be stronger than everyone else. Likewise, throughout Naruto and Naruto Shippuden, Naruto Uzumaki always wanted to be the Hokage. Black Clover's main character, Asta, is no different.
With the goal of one day becoming the Wizard King, his biggest obstacle remains the fact that he's incapable of using magic. On top of that, there are a number of other Magic Knights who are chasing this role as well. While many, like Rill and Yami, would definitely become the Wizard King before Asta, they've set their sights on other pursuits. Only a few mages have openly declared Asta as their rival, meaning that he has to go through them and shatter their dream of becoming the Wizard King in order to fulfill his own.
5 Leopold Vermillion
As Black Clover progressed, the discrimination present in the Clover Kingdom between peasants and nobles became more and more evident as Asta and Yuno continued to face backlash from their colleagues despite already proving their worth on the battlefield. Leopold Vermillion stood out from other nobles, having declared Asta as a rival even before he had the chance to see him in combat.
While it's clear that Asta has bigger fish to fry when it comes to Magic Knights to surpass on his journey to becoming Wizard King, his rivalry with Leopold has only continued to motivate the youngest Vermillion sibling to further improve his skills. Given all the work he's been putting in, there's no doubt that he will soon become worthy competition for Asta.
4 Mars
Mars might be from the Diamond Kingdom, but he's no stranger to the Clover Kingdom, having partaken in multiple invasions into enemy territory. When Mars made his first appearance, his Flame and Crystal Magic made him unbeatable, so much so that even Yuno and Klaus couldn't touch him. Only Asta's unique Anti-Magic swords were able to beat him. This loss changed Mars, causing him to regain his memories and therefore making him a softer, more compassionate individual.
While he can never become the Clover Kingdom's Wizard King, he and Asta both share similar goals. When Mars was last seen in the anime, he departed so that he could take control over the Diamond Kingdom, reforming it into a peaceful nation. This is similar to Asta's goal since he wants to become the Wizard King and eradicate the discrimination that exists between those blessed by mana and those who aren't as fortunate. With mage making a promise to be there for the other when he needs help, they essentially made a pact to continue training so that in the case one actually does need the other, they can pick up one another's slack.
3 Yuno
While Yuno might not be Asta's strongest rival, there's no doubt that he'll soon become that if he continues to progress at the rate that he has. In just a single year, he's become a 1st class Senior Magic Knight and has even become the vice-captain of the Golden Dawn. Not only was he chosen to wield a four-leaf clover grimoire, but he was also blessed with spirit magic, being able to wield the wind spirit. Yuno's accomplishments continue to push Asta to train hard since Asta knows that it won't take long for him to fall completely behind Yuno.
What makes their relationship so special is their identical upbringing: both were raised as orphans in Hage. But while Yuno seemed to have everything going for him, Asta had to work twice as hard just to get half of Yuno's praise. Even after all he's done, many Magic Knights still doubt Asta's abilities, but even before he received his grimoire, Yuno never did. His snide comments only serve to motivate Asta, but it's clear that he respects him more than most. It's clear that Yuno and Asta will one day be forced to duel for the title of the Wiard King, which will undoubtedly result in one of the greatest fights in the series.
2 Fuegoleon Vermillion
Unlike many royal families, the Vermillions have been shown to overlook status, and instead, see an individual's worth as proportionate to their strength and potential. After seeing Asta in action during Rades's attack on the Royal Capital, Fuegoleon admitted that Yami's foresight was better than anyone else's, having seen Asta's value when no one else did. When Asta was struggling to defeat Rades, Fuegoleon used his powerful Flame Magic to defeat him, showing Asta just how strong he would have to get if he hoped to one day become the Wizard King.
While Yuno is Asta's most prominent rival, Fuegoleon is one of his strongest, and with Salamander, the spirit of fire, now under his belt, there's no way Asta would be able to take him on head-to-head just yet. He would not only have to sharpen his skills with his Anti-Magic swords, but he would also need to perfect his Black Asta form since it presents his only chance of surpassing Fuegoleon in terms of raw power.
1 Nozel Silva
While Nozel Silva possesses a few likable qualities, there's no avoiding the fact that he's played a role in the discrimination of commoners, especially those lacking the extensive mana reserves that he and his siblings had been blessed with. When the Clover Kingdom was under attack by the elves, Nozel showed little compassion for commoner villages like Hage that were being ravaged, refusing to even stop to help Yuno and Asta save their hometown. As the series progressed, fans learned that he treated Noelle with such disdain simply, because that was the only way he thought to discourage her from joining the battlefield, although this did more harm than good for her psyche in the long run.
His rivalry with Fuegoleon was acknowledged early on in the series, and after Fuegoleon was maimed by Patolli, Nozel was determined to finish the dark elf off to avenge his rival. With Asta and Yuno right behind him, he proclaimed that he would be the one to become the Wizard King. Neither boy could argue with him, having seen just how superior his magic was to theirs (and even Fuegoleon's, although this changed after Fuegoleon inherited Salamander). It's almost frightening to think that Asta will have to get strong than him if he wants to one day sit on Julius's throne.