Luanda Airport Car Rental is a new portal that links to a number of car hire operators in Angola. I have not used the site so cannot verify how reliable it is.
First time visitors to Luanda would be well advised not to hire a car on arrival. The roads are chaotic, traffic is very slow, there are few decent maps and even fewer road signs. Instead I strongly recommend you ask your hotel if they will provide a transfer service or contact one of the local travel agents listed in the Bradt Guide to Angola.