Its hard to say who is the cheapest as I think it really depends on the dates. I am going to SA in March/April and Air France were the cheapest.
The most useful website I found was www.itasoftware.com. It allows you to search for the best fares over 30 days, so you can pick out the best option. You need to say how long you expect to stay so it can incorporate the return journey into the price. Just sign in as a guest. You can then go to the airline's own website and book there.