Vodacom, particularly given the recent MTN billing fiascos. Also depends on what you want to do: mainly data or voice. Data is much cheaper from Vodacom since their recent bundle-size increases for the same price -- it's actually surprising MTN hasn't felt forced to match this. And you can 'work' the prepaid system a little better with Vodacom if you know the nuts and bolts, e.g. knowing that if you buy a R1,000 airtime voucher you will actually get R1,100 airtime, knowing you can convert Talking Points to call discounts, sms bundles or even data bundles.
Or of course you could live dangerously and try Virgin.....