I don't have a recommendation but I was wondering the same question and checked out wordai's website. If they're the best why do they use the same articles for samples each time? Their turing plan sample is pretty impressive, but you can't compare it the original, and towards the end the quality of the writing degenerates. There's even a nonsense heading.
The best results will be from manual spinning in my fairly limited experience. If you're a reasonably fast typer and really care about quality a lot of spinning software might actually slow you down compared to a rewrite. There aren't all that many human's checking websites though, and generally they're bored, tired, rushed, and underpaid anyway.