Just thought I would share this with the forum as many people have asked me this question.
The best investment account at the moment is Absa's Investment Advantage Account. With this account, you earn 4.5% interest per annum and you have access to your money 24/7 (no waiting).
The second best is Nedbank's offer with 5% interest per annum and access to your money after 24 hours waiting period. Other banks offer similar interest rates as well, though you have to wait a certain period in order to gain access to your money.
I will keep this thread up-to-date with the most recent accounts that banks are offering.