When the key criteria for leadership selection is alignment with a specific ideology, and that ideology is defined by a specific person, it's almost tautological that said person will end up in charge for as long as they'd like to be.

Mao Zedong: 1943 - 1976 (various titles?). He was sidelined around 1959 and staged a successful coup, retaking actual power, in 1966.

Hua Guofeng: 1976 - 1981

Hu Yaobang: 1981 - 1987

Zhao Ziyang: 1987 - 1989. Removed from office for reasons related to the Tiananmen protests.

Jiang Zemin: 1989 - 2002

Hu Jintao: 2002 - 2012

Xi Jinping: 2012 - present

( https://en.wikipedia.org/wiki/Leader_of_the_Chinese_Communis... )

So far he's in third place for length of service.