Statistically Michael Schumacher is the best, though it's not an apples to apples comparison as the car is a much larger factor in driver success than it has been in the past.  My vote's with Ayrton, he had a razor sharp focus that I haven't seen since, and F1 in the early 1990's was much more competitive than it is today.
Who is the best formula 1 driver to ever live?