Botswana's former President Festus Mogae has won a $5m (£2.8m) prize to encourage good governance in Africa.
Mr Mogae, who stepped down in April after two terms in office, said he was honoured and humbled by the award.
Botswana is one of Africa's most stable countries - it has never had a coup and has had regular multi-party elections since independence in 1966.
Announcing the prize, ex-UN head Kofi Annan also commended Mr Mogae for his action to tackle the Aids pandemic.
The Ibrahim Prize - the most valuable individual annual prize in the world - was set up by Sudan-born telecoms entrepreneur Mo Ibrahim.
As well as the $5m prize, Mr Mogae, 69, gets $200,000 a year for the rest of his life.
"One does one's work, one uses one's best endeavours to do a job as well as one could, and if other people then assess it and judge it to be meritorious and worth of recognition it's then honouring and humbling," Mr Mogae told the BBC.
However, he also pointed out that Botswana was already doing well before he became president in 1998.
"I did not create the democracy in my country, I consolidated it and deepened it by practiced, accountable governance, respect of the rule of law, independence of the courts, respect for human rights, including women's rights," he said.
But Mr Mogae also inherited a country with one of the world's highest rates of HIV/Aids and he took strong action to tackle it, making Botswana the first sub-Saharan African country where anti-retroviral drugs were widely available for free.
The drugs are known locally as "Mogae's tablets", reports the AP news agency.
"President Mogae's outstanding leadership has ensured Botswana's continued stability and prosperity in the face of an HIV/Aids pandemic which threatened the future of his country and people," Mr Annan said.
Botswana has Africa's highest average income and is seen as its least corrupt country, according to Transparency International.
It is the world's biggest diamond producer but unlike other resource-rich countries in Africa, this has not become a source of conflict.
"Botswana demonstrates how a country with natural resources can promote sustainable development with good governance, in a continent where too often mineral wealth has become a curse," Mr Annan said.
Mr Annan also noted that Mr Mogae had tried to diversify Botswana's economy away from its reliance on diamonds.
In 2006, President Mogae's government introduced a law curbing the sale of alcohol and banning it on Sunday.
He blamed alcohol for the spread of HIV/Aids, among other problems.
But Mr Mogae also came in for criticism from lobby group Survival International for Botswana's policy of relocating Bushmen groups away from their traditional homes in the Kalahari desert.
Mr Mogae was succeeded as president by Seretse Khama Ian Khama in April.
Former Mozambican President Joaquim Chissano won the inaugural Ibrahim Prize last year.