In a multiverse populated by Saiyans, Kais, Androids, and members of the Frieza Clan, there was arguably no species in Dragon Ball Super that could confidently claim to be the toughest. Then Dragon Ball Super introduced Angels to the storyline, and at last one species separated itself from the rest.
Prior to Goku's climactic fight with Kid Buu in Dragon Ball Z, no one knew what an Angel was, or that they even existed. However, beginning in Naho Ooisha's Dragon Ball spin-off Dragon Ball SD, and continuing in Dragon Ball Super, it is revealed that sometime after Kid Buu is defeated, Beerus, the God of Destruction for Universe 7 is awakened, and with his awakening, fans learn of Whis, the Angel assigned to train and teacher Beerus how to use his powers. Over the course of the series and especially during the Universal Survival story arc, which runs between Dragon Ball Super #27 and #42, readers learn that Whis is one of 14 other Angels who work for Dragon Ball's most powerful being, Zeno and Future Zeno to administer the 12 universes and their various Gods of Destruction.
If the simple fact that Angels are the dedicated "God trainers" of the multiverse does not adequately denote just how powerful they are, the additional point that their only pupils are the Gods of Destruction means, at their weakest, they are as powerful as one of the more destructive forces in all of creation. For instance, in Dragon Ball Super #1, Beerus destroys a planet with his fingertip after being served an unsatisfying meal by the leaders of the planet. Beerus can break the laws of physics on a whim. Assuming that every teacher has more expertise and know-how than their students, is destroying a planet is so easy for Beerus, if obvious that Whis could destroy the planet with even less effort.
For comparison, so far, Beerus has yet to be defeated by an opponent in battle. He has also humbled some of the most powerful beings in the universe including the Supreme Kai, and the Super Saiyan God, and master fighters such as Goku and Vegeta. However, despite his win list, Whis has shown an ability to stop Beerus when necessary, such as the time in Dragon Ball Super #5, when Beerus fought with his brother Champa, wreaking multiple worlds, and were only separated by Whis and his sister Vados. While Beerus believes he is stronger than Whis, there is one technique that Whis has taught Beerus, that he has yet to master. That technique, which is known as Ultra Instinct, as a form stronger than Super Saiyan 4, it allows the user to mindlessly react to an opponent's movements as if reading their minds. At present, there is no technique that can overcome it when used by a master-level practitioner such as Whis and other Angels.
That Beerus has not yet mastered Ultra Instinct speaks volumes about both the gap between him and Whis as well as the omnipotence of Angels in the Dragon Ball Super multiverse.