Discovery, been with them for about 7 years, not once have I had any issues with claiming. Most recent claim was 230k 3 years ago and every cent was paid barring some R3.56 item that the doctor claimed, but was not supposed to claim for.
Every medical aid has its pro's and cons. As an example, Bonitas has better cover for pregnancy for scans and checkups, where as this is paid for out your savings account with Discovery. Bonitas will have better cover on some chronic illnesses. Of course these things come at a price so see what you're willing to pay and choose plans from each provider within that price range, then look at differences.
My personal opinion, if I'm involved in an accident, I feel safer being on Discovery than any other provider based on past claiming experiences.