Kumoko. At the very start, her clever tactics involving the terrain and the video game-esque mechanics of the world (e.g., Level-Up Fill-Up every time she's in critical danger) is definitely something to be admired, especially since she begins as a lowly Mook spider that every other monster could kill without trying.
In the fight with Alaba, the moment he spend all his skill points to take skills that are effective against Kumoko is one for Kumoko. It should be noted that, firstly, earth dragons are a race of proud warriors. Also, most monsters don't spend any skill points for their whole life, and in Alaba's case, those are hundreds of years worth of skill point. In other words, Kumoko has made proud warrior consider her worthy of fighting with all his life.
Kumoko discovers a method to survive Abyss Magic. Then she revises it to develop a method of immortality that effectivelyrendersherunkillable for the vast majority of the named characters, to the point even the Demon King is incapable of defeating her even if Kumoko is vastly outmatched.
Once the reader gets far into the story and realizes that Shiro and Kumoko are in fact the same person as well as how this came to be and in just how many places they stuck their fingers into, the reader may tremble at the sheer scale of chessmastering that the author performed in order to intricately connect the web of subplots.
The Queen Taratekt is called a mythical beast of unimaginable power that is on par with the strongest dragons, and while it does get to show off that it can indeed single-handedly trample an entire battlefield on its own, it's not until the Elf Village Battle (Take Two) that we get to see the true extent of its might: a Wave-Motion GunBreath Weapon that can vaporize mountains, reshape the landscape and penetrate into outer space.
In the manga and anime, Kumoko has reached the surface and is contemplating how she's safe from Mother as she's too big to exit the Labyrinth. In response, Mother uses her breath attack to blast an exit through miles of rock and proceeds to chase Kumoko personally.
