Baki:Hanma Baki, Vol.23, Chapter 187 : My Friend

Grappler Baki (グラップラー刃牙 Gurappurā Baki) ― Original series, serialized in Weekly Shōnen Champion from 1991 to 1999. Collected into 42 volumes, that encompasses The Champion, the Kid, and the Maximum Tournament sagas. From 2007-2008 it was collected into 24 deluxe edition volumes.

Baki (バキ, officially romanized as New Grappler Baki: In Search of Our Strongest Hero) ― Second series, also serialized in Weekly Shōnen Champion from 1999 to November 24, 2005. Collected into 31 volumes, and encompasses The Prisoners, the Chinese Challenge, and the Alai Jr. sagas.
This series is licensed for English release by Media Do International, who began releasing it digitally in August 2018. The company stated a future print release is possible and that they are interested in the original manga as well.

Baki Hanma (範馬刃牙 Hanma Baki, officially romanized as Baki: Son of Ogre) ― Third series, again serialized in Weekly Shōnen Champion, began on December 1, 2005 and ended on August 16, 2012. Collected into 37 volumes, and encompasses the Oliva’s Fortress, The Prehistoric Menace, Retsu Kaiou’s Boxing, and the final confrontation of Yujiro and Baki.

Baki-Dou (刃牙道 Baki Dō, lit. “Baki: The Way”) ― Fourth series, serialized in Weekly Shōnen Champion from March 20, 2014 to April 5, 2018.It features Miyamoto Musashi who is challenged by various Baki characters after being revived into modern-day age. Collected into 22 volumes.

Baki Dou (バキ道 Baki Dō, lit. “Baki: The Way”) ― Fifth series, began serialization in Weekly Shōnen Champion on October 4, 2018. It has the same name as the fourth, but with Baki’s name written in katakana instead of kanji. It features Nomi no Sukune.Collected into three volumes.