Even the strongest anime heroes have to have been defeated at some point... right? Not necessarily - there are actually a few anime heroes who have never lost a fight. This might sound like a bad thing - after all, what are the stakes if you always know how things will turn out in the end? For undefeated anime characters who are interesting to watch, what's truly noteworthy isn't the outcome of the battles - it's the inventive methods they use to achieve their goals.
Sora and Shiro of No Game No Life will definitely wipe the floor with any opponent who approaches them, but what strategy will they choose, and how will they implement it? This question still creates tension even if the outcome is clear. Everyone knows that Saitama is going to defeat all of his enemies, and they even know how he'll do it, but what kind of hilarious things will happen in the process? Characters who always win give the creators an opportunity to showcase stakes that are more inventive than simply winning and losing.
All Might isn't the #1 hero in the world of My Hero Academia for no reason - it's because he's never lost to a villain in his whole career. Has he taken some serious damage? Sure - that's why he's no longer able to maintain his muscle form, and why he barely eked out a victory against All For One during their final showdown. But despite the extreme challenges he faced, he managed to end his hero career with a perfect record.
If an anime fan hacked into dictionary.com and uploaded a picture next to the word "unbeatable" it would be a picture of Saitama. By virtue of doing 100 push-ups, 100 sit-ups, 100 squats, and running 10 kilometers every day, he's able to defeat anyone with a single punch - at least, that's how he says he got so strong. He can take out literally anyone - including a meteor hurting toward Earth. How powerful would someone have to be to defeat him? One shudders to imagine it.
Koro-sensei is a tentacle monster with a smiley face for a head, so he doesn't exactly look like he's impossible to defeat. Looks can be deceiving, however. Despite efforts from everyone from the military to a group of middle students that he's personally training to be able to take him out, no one comes even remotely close to beating this guy. He's super strong, moves at incredible speeds, and can use his mucus as a disgustingly effective protective barrier.
Ultimately, Koro-sensei's life comes to an end at the hands of his students, but it's only because he allows it to happen - he wants to be stopped before the anti-matter inside him blows up and destroys civilization. Because he's the one who trained the kids to be able to do it, this counts as more of a victory than a defeat.
Sora and Shiro are an indestructible brother-sister combination... when it comes to video games. Before they're transported to Disboard, this distinction wasn't getting them very far - but in a world where all things are decided through gaming, they quickly manage to rise to power by never once losing a game. The question becomes not if they'll win, but what inventive strategy they'll use to make it happen.