Rasengan

The Rasengan is a powerful A-rank technique invented by the Fourth Hokage, Minato Namikaze, which he created by observing the Tailed Beast Ball. Minato spent three years developing the Rasengan; his plan was to take the shape transformation of one's chakra to the "highest possible point" and then combine it with his chakra's nature affinity, creating a technique that did not need the aid of time-consuming hand seals to control. He died before he could accomplish this, but on shape transformation alone, the Rasengan is still a very useful, powerful, versatile, and difficult technique to learn and master.

Usage
The Rasengan does not require any hand seals to use, relying only on the chakra control of the user. Once it is complete, it becomes self-sustaining, which does not give the Rasengan a definite limit like the Chidori. The compacted nature and moving speed of the chakra allows the Rasengan to grind into whatever it comes in contact with to inflict major damage. This tends to launch the target backward after being hit or, if the target is lying on the ground or being launched to the ground, the destructive force is enough to form a crater. In the anime, Naruto was able to throw the Rasengan, making it a mid-range projectile technique. It has been noted a few times that the Rasengan is superior to its counterpart, the Chidori, by Jiraiya, Kakashi Hatake, and even Sasuke Uchiha after seeing the after effects of the Rasengan on a water tower.

The Rasengan was designed to use the caster's elemental affinities. Minato was unable to add his affinities to his Rasengan before he died, while Kakashi Hatake tried to add his lightning element to his Rasengan, but was unable to and explains that he created the Chidori and Lightning Cutter instead, which require a certain point of shape and nature manipulation itself. Naruto was the first to add his wind affinity to his Rasengan to create the Wind Release: Rasengan, and even advanced it up to the point that he could create the far more powerful and destructive version, Wind Release: Rasenshuriken.