I would argue that he became his greatest in the 90s. Any South African might tell you that as he is their founding father of the new Apartheid-free nation. But I remember when Mandela first was released from prison. He did a tour around the world, and there were fawning fanboys all over the world. Bill Graham Presents, which was a massive concert promoter in the Bay Area, even hosted an event of him speaking at the Oakland Coliseum.
http://www.nytimes.com/1990/07/01/us/mandela-ends-tour-of-us-with-oakland-appearance.html
Now what people projected on to him at the time was his role as a resistance fighter to everything wrong with Apartheid. It was a lot for one person to bear, but he was a symbol.
And yet despite the immense crowds and adoration he got back then here in 1990, I think all that paled in comparison to the role he played right afterwards in founding a new South Africa, establishing one of the most liberal constitutions in the world, and almost single-handedly preventing what was expected to be the bloodiest race riot and chaos in the world under the Truth and Reconciliation Commission.
It is simply astounding what people in South Africa expected at the time, and what he did by example to change the course of history.
He had saved his best act perhaps for last.