A few years ago on Quora, I answered this question. I though you guys might enjoy what my answer was.
In the Naruto series jutsu’s are ranked in six different classes from E to S. This is based on “the difficulty or skill necessary to perform a technique”.
E Ranked – are learned by academy students there the most basic.
D Ranked – are considered Genin level techniques
C Ranked – are considered Chuunin level techniques
B Ranked – are considered Jonin level techniques
A Ranked – are both Jonin and Kage level, many are forbidden
S Ranked – are often secret techniques. They are often typically unique to the user.
Tailed beast abilities, Kekkei Genkai and other abilities are not ranked.
So if your going to choose “the strongest jutsu” most likely will come from S Ranked classed. When using the word “powerful”, it important to determine what that means. Does it mean destructive power, effective power, or healing power. I think all 3 qualify.
7. Wind Release: Rasenshuriken
Naruto uses his wind manipulation to create a “giant fuma shuriken”. When the Rasenshuriken hits his target “microscopic wind-blades that severely damage the body on a cellular level”. It produces so many micro wind blades that the sharingan can’t follow them all. The “wind-blades sever all nerve channels in the body, leaving the target unable to move after being struck”. They attack the entire chakra circulatory system and cannot according to Tsunade “be repaired by any medical jutsu”. Naruto improved this technique by throwing it, on impact it also causes a tremendous amount of damage. Naruto has managed to add a variety of chakra types as well, and increase or decrease its size making it a dynamic jutsu.
6. Tengai Shinei
The user (Madara) summons massive meteorites from space. The impact of this technique was so devastating that it destroyed most of the fourth division and Madara’s own body.
5. Creation Rebirth
Is a medical ninjutsu, created by “the greatest of medical ninja”, Tsunade. She releases chakra stored in her forehead simultaneously, the body’s “cell division is forcibly stimulated by proteins”, “reconstructing all organs and all tissues making up the human body”. Tsunade claims that “as long as she has chakra it is impossible for her to die by any means”.
4. One’s Own Life Reincarnation
Created by Chiyo, The “user, giving their life force to the target, uses all their chakra as an intermediary”. This can be used on the living or the dead. If using the jutsu on the living “can save and restore the target from an otherwise fatal condition but will leave the user greatly exhausted. When used on the dead, the user will find certain death in exchange for the soul of the deceased”.
3. Mangekyu Sharingan
Once the “Mangekyu Sharingan” is obtained a whole host of abilities are released. They range from genjutsu to ninjutsu, but all are powerful.
Susanoo – “is an ability granted to those who awaken the powers of both their Mangekyu Sharingan. It creates a gigantic, humanoid being that surrounds the user and fights on their behalf. As one of the strongest techniques granted to those that have acquired the Mangekyu Sharingan, it is the user’s guardian deity, but at the same time, it consumes the user’s life force and damages their body over time”.
Izanagi – “When activated, the caster removes the boundaries between reality and illusion within their personal space”.
Izanami – “It is a genjutsu which affects the target through physical sensations shared between them and the user. Like Izanagi, in exchange for the temporary ability that it grants the user, the Sharingan with which Izanami is cast is rendered blind and loses its light forever”.
2. Summoning: Impure World Reincarnation
Kabuto claims this “technique to be the greatest technique in the shinobi universe due to it posing no risk at all to the user”. Tobirama admitted “that the technique’s new nature was essentially perfected and able to restore an individual to practically their full power”
The Rinnegan gives “access to a multitude of abilities, such as the potential to master all five basic nature transformations and the capacity to use any technique. The Rinnegan allows the user to see chakra including the chakra points within the body , as well as otherwise invisible barriers. However, the Rinnegan’s field of vision can be blocked by simple means such as smoke bombs”.
Madara could perform “the Limbo: Border Jail technique, creating corporeal shadows which are only visible to the Rinnegan”. Sasuke’s Rinnegan allows “him to shift spaces within a certain range of himself”. Kaguya “she can teleport herself and others to a series of alternate dimension”.