Personally I think its impossible to compare drivers across decardes in F1. Schumacher is a great driver - but in previous years he has had an impecably maintained machine... Schumacher drove a Le Mans car once and said never again - he didn't like it at all...of which many other mentioned below have raced and won in!
Jim Clarke anyone? Senna - definiately, Fangio - of course! Stewart, Mansell, and a host of others some mentioned below...
In motorsport heaven - I'd love to see them all at the wheel of a60's/70's F1 car - see who wins then!! and I'd put money on Schumacher not finishing on the podium!