Out Of Every Ninjutsu In Naruto, This One Stands Above The Rest
In the world of "Naruto," the life of a shinobi greatly depends on their ability to use Ninjutsu. With the notable exception of Taijutsu specialist Rock Lee, every shinobi in the entire series performs Ninjutsu in some shape or form. As the most common of the three major types of ninja technique — the other two being genjutsu (illusion techniques) and taijutsu (hand-to-hand combat) — ninjutsu composes the vast majority of abilities that shinobi use throughout the series. Name a jutsu, and it probably falls somewhere in the ninjutsu family regardless of its similarity to other techniques. A summoning jutsu has almost nothing in common with Naruto's signature Rasengan, yet both are considered ninjutsu.
Another side effect of ninjutsu being the most common brand of jutsu is that the series' shinobi have developed hundreds (if not thousands) of specialties over the years. With so many different jutsu, not all are created equal. In fact, out of every ninjutsu in "Naruto," only one stands above the rest. It's called the Tailed Beast Bomb (or Tailed Beast Balls), and it's so powerful that fans on Reddit consider weapons of mass destruction weaker when it comes to destructive capabilities.
The Tailed Beast Bomb is basically a nuke for ninjas
It's no secret that jinchuuriki house one of the most powerful abilities in the "Naruto" universe. Tailed beasts are perhaps the most overpowered n chakra-wielding creatures in the entire series, and having one sealed inside your body essentially makes you a living weapon. In fact, different nations put great care into ensuring their jinchuuriki stay under their control for the sheer purpose of increasing their military might.
There's a good reason for this magical ninja arms race. By having its tailed beast gather a mixture of negative black chakra and positive white chakra in its mouth, a properly trained jinchuuriki can fire a Tailed Beast Bomb capable of leveling a mountain (via Narutopedia). On top of that, there's no upper limit to how powerful a tailed beast can make the ball. Assuming it doesn't run out of chakra, a jinchuuriki could make a Tailed Beast bomb with enough force to obliterate entire landmasses.
In addition, the Tailed Beast Bomb is versatile enough to be used in a variety of situations. The tailed beast in question can release the energy from the bomb in a variety of forms, including a wave or a beam. If multiple jinchuuriki are fighting together, they can combine their bombs into one. Users can also create a small, hand-sized beast bomb without summoning their beast. Furthermore, a beast bomb can even be combined with other jutsu, such as the Rasengan, for a unique and especially destructive effect.
That's not to say there aren't some worthy competitors
While the Tailed Beast Bomb certainly ranks the highest of all ninjutsu based on its sheer destructive power, ninja conflicts in "Naruto" are often highly situational. In some circumstances, the Tailed Beast Bomb just won't cut it. When that's the case, there are a few other jutsu that compete against the Tailed Beast Bomb for the top spot.
For instance, the "Impure World Reincarnation" jutsu allows the user to bring certain people back from the dead. Once summoned, the resurrected is required to carry out the summoner's command to the fullest. Using this technique, Orochimaru managed to resurrect three of the Leaf Village's most powerful Hokages in his duel against the Third Hokage. While the resurrected Hokages were nowhere near as powerful as they were in life, the three warriors still managed to push the Third to his limits.
Likewise, another jutsu comparable in scope to the Tailed Beast Bomb is the Susanoo. As the ultimate technique available to those who possess a Mangekyo Sharingan, the Susanoo can turn a single shinobi into a one-man army. The Susanoo itself is a manifestation of chakra in the form of a massive warrior surrounding and controlled by the user. It is incredibly durable and can wield its sword for offensive techniques. A piece of armor and a weapon all in one, the Susanoo is ostensibly Sasuke Uchiha's best answer to Naruto's jinchuuriki powers, including the Tailed Beast Bomb. Of course, neither of these techniques are the rarest abilities featured in the show. One move was so specialized that only two characters ever learned it.