In general humans tend to have darker skin (phenotype) when they live closer to the equator. Of there are a lot of variation depending on the ethnicity.

Dark:

Generally speaking, populations who have lived in close proximity to the equator for many generations, will be the darkest. The darkest people I’ve seen were from South Sudan and the surrounding areas in Kenya and such. I’ve also seen dark people form Senegal But there are also dark people in Central Africa, West Africa and East Africa.

Lighter:

Some of the Black South African people tend to be a bit lighter, because they have intermarried with Khoisan in previous generations.

The Khoisan themselves are usually lighter than other indigenous Africans, they have a light brownish to reddish complexion.

The Coloureds of Southern Africa, who are of mixed Khoisan, European, South Asian and Black African descend are also often very light.

In Northern Africa and the Horn of Africa Region you have various lighter skinned people, such as Nubians, Arabs, Berbers, Somalis, some of the Ethiopian people and Tuareg.

You can see on the following map the human skin colour distribution of natives population before 1940. Of course this map is far from perfect. It was made by comparing skin to a chromatic scale.

However it gives us some indication of geographical differences.
From my own experiences and after a couple of googleing it seems that darkest ethnicity in Africa are people from the Nilotic ethnic group, and especially Dinka people who are mostly in South Sudan

Natives of Buka and Bougainville at the northern Solomon Islands in Melanesia and the Chopi people of Mozambique in the southeast coast of Africa have darker skin than other surrounding populations. (The native people of Bougainville, Papua New Guinea, have some of the darkest skin pigmentation in the world.)

Around 3–4 populations can be mentioned if we are talking about truly skin so dark it appears black (BTW these populations are not necessarily related either, having incredible amounts of melanin doesn’t mean they have to be related, some of these populations are more genetically distant from each other, than to even a European person is to an African).