For the worst driver, I do absolutel NOT agree with Badoer. Badoer was a better driver in lower classes than Barrichello, but had the bad luck of never making it to a top team. And when he did, he hadn't raced for over 10 years. Not really fair to judge him on that. Fisichella couldn't do it much better in that same Ferrari, did he?
For the worst driver of all, I come out with two names: Yuji Ide and Jean-Dénis Délétraz. Don't know who was the worst of the two, I suspect the Frenchman...
For the best driver who didn't get into Formula 1, I think about Arie Luyendyk. Could have been the first Dutch World Champion if he got himself a good car. He was in talks but not really interested, as far as I know.