Killua is the best friend of Gon Freecss and one of the main characters in the Hunter x Hunter story. Being a member of the Zoldyck family, Killua is blessed with immense talent and skills as a fighter.
Over the years, he's honed his skills to a point where even seasoned Hunters fail to keep up with him. One of Killua's strongest abilities is known as Godspeed that allows him to respond to all incoming threats automatically and move extremely fast. Although Killua looks invincible in this form, there are some characters who are stronger than he is and can, therefore, defeat him.
10 Can Defeat: Netero
Isaac Netero was one of the most powerful Hunters to ever exist in the series. Known to be the strongest of them all in his prime, Netero mastered Nen to a point where even Meruem couldn't help but praise his skill.
Netero's powers went beyond the human limitations and even though Killua could possibly evade some of his attacks, there is only so much he can take before finally falling to the 100-type Guanyin Bodhisattva.
9 Can't Defeat: Gon Freecss
Gon is the best friend of Killua Zoldyck and the main protagonist of the Hunter x Hunter series. His ability as a fighter is just as good as Killua's and he is known to carry great potential. Although Gon and Killua have always been relative in terms of power, Killua seems to have pulled ahead with Godspeed.
Furthermore, Gon isn't able to use Nen anymore, meaning he is absolutely no match for Killua at this point.
8 Can Defeat: Meruem
The strongest known character in Hunter x Hunter, Meruem was the King of the Chimera Ants. Also known as the pinnacle of evolution, Meruem was stronger than any Hunter in existence, including Isaac Netero.
The latter put up a decent fight against him, however, the difference in skill was just too massive. Killua stands absolutely no chance against Meruem.
7 Can't Defeat: Leorio
Another one of Gon's friends, Leorio is a Hunter who aims to become an excellent doctor. From the main group of characters, it goes without saying that Leorio is the weakest of them all.
He has barely learned to use Nen and it'll take him quite some time to catch up to the rest of the group. When compared to Killua, Leorio is nowhere close to even putting up a fight.
6 Can Defeat: Illumi
Illumi is Killua's older brother and an excellent assassin, as expected of a Zoldyck family member. Illumi's skills as a Manipulator are known to be top-notch and even Killua fears the power in his aura.
As strong as Killua is, he cannot compare to Illumi right now. However, in time, he'll definitely be on the same level as Illumi, if not higher. For the time being, he doesn't stand much of a chance against his brother.
5 Can't Defeat: Genthru
One of the main antagonists of the Greed Island arc, Genthru was the man renowned throughout the game as the Bomber. He has an incredible Nen ability that made his aura explosive and allowed him to plant bombs.
There is no denying that Genthru was a menacing enemy, however, Gon was able to defeat him, which means Killua can do the same. Furthermore, Killua has grown much stronger than he was during the Greed Island arc.
4 Can Defeat: Menthuthuyoupi
One of the Royal Guards of Meruem, Menthuthuyoupi was a monster in combat, stronger than several seasoned Hunters put together.
Killua traded blows with Youpi in his Godspeed mode and while he did evade his attacks and landed a bunch on him, the difference in skill between the two was massive. Killua cannot maintain his Godspeed mode for long, and even if he could, the damage he deals to Youpi would be rather insignificant.
3 Can't Defeat: Hanzo
An excellent Hunter, Hanzo has played a role in some of the arcs in the series and proved himself to be a great Nen user. In the Hunter Exams, Hanzo emerged to be stronger than Gon Freecss, however, he lost the fight due to the latter's determination.
He's likely to have grown a lot since then and become a powerful Hunter, however, it is highly unlikely that he's stronger than Killua in his Godspeed mode.
2 Can Defeat: Hisoka
Hisoka is a former member of the Phantom Troupe who bore the #4 tattoo. As a Hunter, Hisoka is tremendous, and his usage of Nen is outrageous as well. Hisoka's aura is known to have the properties of both rubber and gum, hence the name Bungee Gum.
That, when combined with several other of his skills, makes him an elite fighter. Killua, even in his Godspeed mode, will certainly not be able to defeat Hisoka.
1 Can't Defeat: Kalluto Zoldyck
Killua's younger brother, Kalluto is also a famed assassin of the Zoldyck family who serves as a member of the Phantom Troupe. He is known to be an excellent fighter, however, his skills pale in comparison to both Killua and Illumi.
Kalluto realizes the difference in skill between him and his brothers and it's easy to see that there's absolutely no way for him to defeat Killua, especially in Godspeed mode.