I would say Telecel - their 3G coverage can't match Econet, but their connection is a bit more consistent. Plus, if you have a lot of people who use Telecel, remember that they do still offer that bonus. So if you buy $1 airtime, you get $1 bonus for calling Telecel to Telecel. Which mean if you buy $1 and turn it into data, there still $1 to call your Telecel contacts; so that's basically FREE INTERNET - providing you know a lot of Telecel subscribers of course.
So the best? In my book, its the one which is FREE!