The Most Powerful Member Of The Seven Deadly Sins Isn't Who You Think
The Seven Deadly Sins revolves around a holy war waged by the Demon Clan upon all manner of other creatures. This latter group includes fairies, giants, humans, goddesses, and more. Magic is common in this world, giving rise to special, combat-oriented magical brigades that beat the Demon Clan back as they try to escape their imprisonment within another realm. The Clan will stop at nothing in their quest for vengeance on Britannia.
Counterintuitively, these demons are named the Ten Commandments, while the good guys are called the Seven Deadly Sins. There are, as you'd expect, seven of them, and they're named appropriately. Meliodas, the Dragon's Sin of Wrath, is the leader of the Seven Deadly Sins, but was born the son of the Demon King and once lead the Ten Commandments. That all happened before he had a change of heart and embraced goodness, however. Through most of the show, it's pretty obvious that Meliodas is the most powerful member of the Sins: He seems to crush his enemies with little more than a flick of the broken sword he carries. So when he goes dark in the third season of the anime, the Sins have a tough time trying to stop him. Yet one of them succeeds.
It's a toss-up
In the third season, Meliodas goes all demon-y, and things look bad. Merlin, the Boar's Sin of Gluttony, traps him inside of a cube with Escanor, the Lion's Sin of Pride. The cube contains magic, so no one outside of it is can be hurt as collateral damage. Escanor is a pretty powerful dude who draws his strength from the sun, much like Superman. He's the only one who has a shot at trying to tame the leader of the Seven Deadly Sins and not dying in the attempt. Thus, the two do battle.
As noon rolls around, Meliodas tells Escanor, "I'm warning you: Come at me again like that, and you'll suffer." At this point, Meliodas has been wrecking Escanor. "Well, I hate to admit it, but you're absolutely right," Escanor responds humbly. "You are quite powerful. I'd say you're second only to me." At that moment, the sun hits its peak, and its dazzling rays empower Escanor fully. His muscles bulge, his pride is reignited, a burning fire surrounds him, and the ground crumbles beneath his blazing feet. He is, for that one brief moment, "The One." At least, that's what Merlin says, and she's a pretty smart cookie. The fight resumes. Escanor, now temporarily invincible, wipes the floor with Meliodas without breaking a sweat.
Now, it's entirely possible that Escanor only beats Meliodas because the former leader of the Ten Commandments underestimates his power. When Meliodas is at full demonic strength, he's cocky, and that arrogance makes him blunder. But still, during that one minute of each day when the sun is highest in the sky, Escanor can take down a fully powered Meliodas with a single stroke. He makes it look like child's play. Fans aren't going to soon forget that — and neither is Meliodas.