I personally think Gani Fawehinmi fits the profile .
"With his boundless energy he tenaciously and uncompromisingly pursued and crusaded his beliefs, principles and ideals for the untrammelled rule of law, undiluted democracy, all embracing and expansive social justice, protection of fundamental human rights and respect for the hopes and aspirations of the masses who are victims of misgovernment of the affairs of the Nation.

He was a notorious critic of the Babangida and Abacha military dictatorships, for which he was imprisoned multiple times" Wikipedia....
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Who would you consider to be the greatest Nigerian of all time and why

OBJ

My mother (no pun intended).

None!

OP, there is none

There are no greatest. Just tribal champions.

The Man is the one and only CHIEF JEREMIAH OBAFEMI AWOLOWO, few of his undeniable and irrefutable achievements are as follows:

1.) He named Nigeria’s national currency as ‘Naira’
when he was the Federal Commissioner for Finance.
The late sage took the name of Nigeria and collapsed it
as ‘Naira’
2.) Awolowo so much loved education and understood the value and importance, he introduced FREE primary education for all and free health care for children in the Western Region. His free education policy was enjoyed even by many of the living elites we have today.
3. )He established the first Television station in Africa in
1959 even before some European countries have Televisions.
4. )He also erected the first skyscraper in tropical
Africa: The Cocoa House (still the tallest in Ibadan).
5.) The first and only non-president to be honoured with the title of GCFR. Awo as fondly called was conferred by President Shehu Shagari with the title of the Grand Commander of the Federal Republic (GCFR) in recognition of his sterling qualities and contributions to the service of the country.
6.) He was the first individual in the modern era to be
named Leader of the Yorubas (Yoruba: Asiwaju Omo
Oodua), a title which has come over time to be
conventionally ascribed to his successors as the
recognized political leader of the Yoruba peoples of
Nigeria.
7.) The man widely believed by admirers to be the best
President Nigeria never had was the first Leader of
Government Business and Minister of Local Government
and Finance and first Premier of the Western Region
under Nigeria’s parliamentary system, from 1952 to
1959.
8.) In 1949 he founded the Nigerian Tribune, the oldest
surviving private Nigerian newspaper, which he used to
spread nationalist consciousness among his fellow
Nigerians.
9.) The party he founded, Action Group was the FIRST to
move the motion for Nigeria’s independence in the
federal parliament and he obtained internal self-
government for the Western Region in 1957.
10.) The University of Ife was renamed Obafemi
Awolowo University on 12 May 1987 in honour of Chief
Obafemi Awolowo first premier of the Western Region of
Nigeria, whose brainchild the university was.

Chief Obafemi Awolowo not only had a foresight for the future, he also had the intellectual capacity to build a future for the coming generation of Nigerians irrespective of tribe and religion, but he was surrounded by nationalists who probably had a different intention of grabbing power for personal gains. Had it been he was able to become the President, Nigeria would have been on a far advance and better pedestal in terms of social, educational, technological and economic development.

Baba Ikenne, Awolowo

At least we SW'ners are proud of his achievements in the region

Dr. Nnamdi Azikiwe. He was our first president, a great nationalist and most importantly, he was de-tribalized.