The former US ambassador to the United Nations, Richard Holbrooke, once described Kofi Annan as "the best secretary general in the history of the UN".
That however was the view of the representative of a Democratic US administration. The Republicans have been far less sympathetic.
Mr Annan is widely seen as an independent leader
Indeed, there was a near-open break in September 2004 when, in a BBC interview, Mr Annan declared about the invasion of Iraq, an issue that has dominated the last years of his time in office: "I've indicated that it was not in conformity with the UN Charter. From our point of view, from the charter point of view, it was illegal."
The 2003 Iraq invasion is not a time Mr Annan looks back to fondly, recalling it as a "depressing period", and one which exposed many flaws in the world body - shortcomings he tried to tackle during his remaining time in office.
Born in Kumasi, Ghana, in 1938, Mr Annan studied in Kumasi, Minnesota, Massachusetts and Geneva before joining the UN in 1962 as an administrative and budget officer with the World Health Organization.
He has served with the UN Economic Commission for Africa in Addis Ababa, the UN Emergency Force in Ismailia; the Office of the UN High Commissioner for Refugees (UNHCR) in Geneva, and at the UN Headquarters in New York where he was head of Peacekeeping Operations.
He became secretary general in 1997 after the US had firmly declared its intention to veto a second term for Boutros Boutros Ghali.
Mr Annan faced some formidable challenges when he first came to office, not least the fact that the organisation was approaching bankruptcy.
After a trip to Washington to urge repayment of dues, Mr Annan's first major initiative was his plan for reform: Renewing the United Nations. He streamlined the UN bureaucracy, cutting 1,000 of 6,000 positions at its New York headquarters.
Aside from his difficulties over the Iraq issue, the secretary general is widely admired for his efforts on behalf of Africa, where the problems of war, famine, disease, and the displacement of millions of civilians continue to blight development and progress.
He has shown personal commitment to tackling the Aids epidemic, teasing money out of the coffers of the world's richest nations and persuading many countries, particularly in Africa, to recognise the grave threat that Aids and HIV infection pose to their future.
In 2001, Mr Annan and the UN received the Nobel Peace Prize.
The judges said: "The only negotiable road to global peace and co-operation goes by way of the United Nations. (Mr Annan) has been pre-eminent in bringing new life to the organisation."
He was however criticised in a report for the mismanagement of the oil-for-food programme under which Iraq, under sanctions, was allowed to sell oil for food and medicines. The report, by the former head of the US Federal Reserve Paul Volcker, said that Saddam Hussein had been left to rake in kickbacks and illegal profits.
Kofi Annan was cleared of helping his son Kojo who worked for a company that won the contract to monitor the programme.
He has also been criticised for not acting more urgently in the crises in Bosnia and Rwanda. He was head of the UN peacekeeping operations when the Srebrenica and Rwanda massacres took place.
Mr Annan's major project at the UN was reform. In a speech in September 2003 he said that the UN was at a "fork in the road".
He pressed for a new philosophy - that of intervention. The UN must place itself above the rights of sovereign states when necessary to protect civilians from war and mass slaughter, he declared.
He appointed a panel of "wise men" who drew up a report agreeing that the UN should assume a role when a state had failed in its "responsibility to protect" its citizens.
In September 2005, a UN declaration stated that "every sovereign government has a 'responsibility to protect' its citizens and those within its jurisdiction from genocide, mass killing, and massive and sustained human rights violations."
The application of this principle remains to be worked out in practice but the principle itself might be Kofi Annan's most important legacy at the UN.