Dragon Ball

The Dragonball continuum consists of two canonical arcs - Dragonball and Dragonball Z - and one arguably canonical anime arc, Dragonball GT. Its story centers around the adventures of Son Goku and his friends (the Z Fighters) as they defend the Earth from villains seeking to rule, conquer, or destroy it. More often than not, these battles result in the death of at least one of the members of the Z Fighters, and so the use of the titular dragonballs is often needed in order to resurrect the fallen. Unfortunately, the dragonballs will only bring a person back to life once; following this, the person cannot be resurrected by the same set of dragonballs. This limitation and the need to resurrect characters barred by it led to a portion of the Z Fighters traveling to Namek to make use of that planet's set of dragonballs.

GT Controversy
There is a large dispute among the fanbase as to whether or not Dragonball GT can be considered canon. While the series did not first debut in the manga format (unlike its predecessors), meta-series creator Akira Toriyama did oversee the series' production and drew rough sketches for the series title image and several of the new characters introduced in GT. He also drew most if not all of the character designs for those characters who came into the series from its predecessors (such as Son Goku), and seems to harbor positive feelings for the series.

Opponents of GT's canonicity claim that as it did not appear first as a manga and possesses several inconsistencies with its predecessors (although it is interesting to note that, of the three series, GT has the fewest and arguably smallest inconsistencies), it should not be considered canon. Some also claim that Akira Toriyama did not technically write the series, despite having overseen its production. Whether or not this claim has any merit appears to be up to each individual, as finding information to back the claim or render it untrue is rather difficult.