It depends on your area of interest. If you are into history, culture and slavery with a litle bit about wildlife and fauna, GHANA would be your best option.
If you are into History and civilization, look towards Egypt, Ethiopia.
If you are looking for wildlife and some plants, look towards the East [Kenya and surrounding countries].
For 2 sharp distinct differences, look towards South Africa. Hope you have heard of the Sun city. There are area for the poor as well. On the whole it is a great place to visit. Great culture, History etc.
This is not to say the other countries are not good. They are but I might not know all, and most of them have just a little of everything.
Do some research yourself.