Who is the world leader who has served the longest (so far) being still alive and serving?

How does he rank among all world rulers in time?

Obviously, Fidel Castro, who has outlived or outlasted nine U.S Presidents

No sorry SR I have someone else. Here's why!
Castro was prime minister of Cuba from 1959 to 1976 and has been president of the government and First Secretary of the Communist Party since 1976. But because of that "switch' in Politics, he cannot be involved as a "non stop world leader" in time.

From Wikipedia -

Fidel Alejandro Castro Ruz (born August 13, 1926) is the current President of Cuba. After commanding the revolution that overthrew Fulgencio Batista in 1959, he held the title of Prime Minister[1] until 1976, when he became president of the Council of State as well as the Council of Ministers. Castro became First Secretary of the Communist Party of Cuba in 1965, and led the transformation of Cuba into a one-party socialist republic.

From the CIA World Factbook -

Cuba

chief of state: President of the Council of State and President of the Council of Ministers Fidel CASTRO Ruz (prime minister from February 1959 until 24 February 1976 when office was abolished; president since 2 December 1976); First Vice President of the Council of State and First Vice President of the Council of Ministers Gen. Raul CASTRO Ruz (since 2 December 1976); note - the president is both the chief of state and head of government
--------
Your answer may depend on the definition of "world leader" but I don't see that anyone other than Castro can be called the leader of Cuba, not since he overthrew Batista in 1959.

According to this site ,Castro is #3 and is considered to be ruling since 1976 only.Source
While Gabon President Omar Albert-Bernard Bongo has been ruling since April 14, 1967. But I must admit, as you mentioned, that the term "leader of the world" is a challenging one. Somehow the change of Castro's politics made him start from scratch all over again from 1976.

It really wasn't a change of politics. The structure of the Cuban government was re-organized. Castro remained in power at all times.
--------
We just discussed the longevity of world leaders recently om AP, but I don't remember in what thread.

Then maybe we should consider the Queen of England or the King Of Thailand? It is confusing when you think about it.

Neither really rule their country.

CIA on Thailand -

Executive branch: chief of state: King PHUMIPHON Adunyadet (since 9 June 1946)
head of government: Prime Minister THAKSIN Chinnawat (since 9 February 2001) and Deputy Prime Ministers CHITCHAI Wannasathit (since 11 March 2005), SOMKHIT Chatusiphithak (since 11 March 2005), SUCHAI Charoenrattanakhun (since 31 October 2005), SURAKIAT Sathianthai (since 11 March 2005), SURIYA Chungrungruankit (since 2 August 2005), SUWAT Liptapanlop (since 2 August 2005)