In Dragon Ball Z, Majin Buu proves himself to be one of the most powerful opponents the Z fighters have ever gone up against. He has seemingly inexhaustible reserves of strength, stamina, regeneration, and magic. He pushes all of Earth's greatest warriors past their limits in their efforts to stop him. He is only defeated when the citizens of Earth pool all the energy they have into one final attack. With such credentials, it sounds like Buu would be a formidable warrior even in Dragon Ball Super.
Sadly, Buu doesn't really get to show what he's made of in Super. Whenever he has a chance to participate in an important fight, he falls fast asleep and nothing can wake him. This could lead fans to believe that Buu is useless, but this is far from the truth. Majin Buu, as in Dragon Ball Z, has the potential to be one of the most powerful fighters in the multiverse.
It's easy to forget just how powerful Buu really is. His ability to wipe out entire galaxies makes him a threat to the entire universe. Not only that, but the potency of his regeneration makes him nearly indestructible. He can destroy a planet while he's still on it, get blown to pieces with it, reform his body, go to another planet and repeat the process as many times as he likes.
He doesn't seem to get exhausted either. In the "Buu Saga", he fights consecutive battles with Earth's strongest warriors and shows few signs of fatigue. His instantaneous regeneration means he doesn't even need rest to recover. He might not even need to sleep. The little sleep he does get only lasts for minutes at a time and it's clearly not a matter of restoring energy. Buu's bottomless reservoir of stamina is part of what makes him nearly invincible.
Even if Buu isn't the strongest fighter, he has plenty of magic abilities that would make him helpful. Not only can he regenerate, but he can also quickly restore his allies to full health. This gives Buu the potential to function perfectly in a supporting role. He can tank hits, heal injuries and even dish out damage as needed.
Buu also has a number of miscellaneous abilities that make him difficult to deal with. Besides what's listed above, he can reshape his body, mimic techniques, and make copies of himself. Even if an opponent is somehow too strong for him, he can either turn them into sweets and eat them or absorb them into himself to make their strength his own. Whoever fights Buu will have a hard time keeping up with all his different powers.
Unfortunately, Buu's fights in Dragon Ball Super aren't good for measuring his fighting ability. His first big fight is against Beerus, the God of Destruction of Universe 7. This match is a blowout because Beerus is, by definition, the most powerful fighter in the universe (it's a job requirement). The fact that Buu loses this fight is an indisputable truth, but the same holds true for anyone who has gone up against Beerus in the series thus far.
Buu's second match in the series against Basil also says little about his power, but for the opposite reasons. Not only is this an exhibition match, but Buu is up against a fighter from the relatively weak Universe 9. His opponent is someone who ends up losing within the first few minutes of the Tournament of Power. If Buu had competed in the Tournament of Power as intended, he could have likely distributed the same embarrassing defeat to many of the other subpar fighters.
It makes sense from a writing standpoint to keep Majin Buu out of the action. The writers must know his overwhelming power and virtual immortality makes him too much for the average fighter to handle. The decision to keep him away from the tough battles so that the Z Fighters who have an actual chance of losing can beat the odds and eke out a win is likely purposeful. Majin Buu would be far too useful for there to be any tension for Dragon Ball Super or any version of Dragon Ball for that matter.