Telkom Mobile is the best mobile data provider by far...provided you have coverage in the areas you travel or reside in - not only for it's one byte billing session, but also for the pricing of it's data bundles. I am lucky enough to live and travel in areas that have excellent Telkom Mobile coverage and speeds. My advice would be to buy a prepaid TM SIM card and check the areas you typically travel in to see what the coverage is like.
Second best would be Afrihost, top notch customer support and data bundle pricing
PS. I have a Vodacom contract which will be cancelled when it expires, an MTN contract that will be cancelled when it expires in November, a prepaid Cell C account which will become my main voice/SMS account when my MTN contract is cancelled, and 40 GB per month data contract with Telkom Mobile (2 x promo 5 contracts with 10 + 10 GB data each).
I use the Telkom Mobile contract for my home data, or for travel (inside a MiFi), and I have a TM SIM in my iPad which is linked into my 40 GB contract - fantastic that they have provided this service...for a few rands a month connection fee, my iPad sucks data off my 40 GB contract