Kid Buu (魔人ブウ 純粋, Majin Bū Junsui), also known as Majin Buu (魔人ブウ, Majin Bū), is the original and purest form of Majin Buu and the last form of Buu that appears in Dragon Ball Z. This Buu appeared after all of the fighters that he'd absorbed were removed from his system. Unlike his previous forms, this form of Buu's thought process is seen to be irrational and spontaneous, even destroying his own body to destroy the Earth.
The first form he appeared in was actually the form that he took after he had absorbed Southern Supreme Kai and Grand Supreme Kai millions of years earlier. Although he appears smaller than any other form, he is more dangerous than any other forms of Buu, due to his full relentless evil and unpredictability. He is also completely free of restraint and has no trace of sanity, and because of this is extremely dangerous to be around. He retains his childish personality, becoming amused and laughing hysterically at his own destruction of entire worlds. In addition to this, he is the final villain of the Dragon Ball manga.
PowerEditIt is unknown if Kid Buu's power is under or over Super Buu's power, but Goku stated that even he could kill Buu if he was at full power Super Saiyan 3. Although it is for sure he is stronger than Majin Buu and Super Saiyan 2 Vegeta. He also seems to be stronger in his normal form than when he absorbed South Supreme Kai. Buu was able to beat Goku on the Kai's planet due to Goku's energy being drained very quickly by the Super Saiyan 3, as he cannot maintain the form for very long outside of Otherworld.
Many fans seem to spectulate that Kid Buu was the strongest of all of Buu's forms, this is not true as Kid Buu is the second weakest form of Buu.