According to Reuters, it’s the Second Congo War (1999-2003) aka Africa’s World War and the Great War of Africa:

(Reuters) –
War, disease and malnutrition are killing 45,000 Congolese every month
in a conflict-driven humanitarian crisis that has claimed 5.4 million
victims in nearly a decade, a survey released on Tuesday said.

The International Rescue
Committee (IRC), which carried out the study with Australia’s Burnet
Institute, said Democratic Republic of Congo’s 1998-2003 war and its
aftermath had caused more deaths than any other conflict since World
War Two.

“Congo’s loss is
equivalent to the entire population of Denmark or the state of Colorado
perishing within a decade,” George Rupp, president of the aid group,
said in a statement.

Despite the nominal 2003 end of the war, the horror continues.  After a civil war or two, even Mobutu looks good by comparison.

Update: Can anyone point to credible estimates of Mobutu’s body count?  He’s named on this list of top mass murderers, but in the victim column there’s a question mark instead of number.