Senna and Prost both had each other to contend with as team-mates, as well as Mansell and Piquet (who really should also be on the list of choices imo) unlike Schumacher who had the like of Jaques Villeneuve and David Coulthard as his main antagonists, for example, if Prost had done a Schumacher and been given no 1 status at McLaren he would probably been a 7 or 8 time champion, yet he wasn't as good as Senna. And Jim Clark was the best of his era, an era I believe had some of the best drivers because motor racing was far more readibly accessable to ordinary folk as it was a hell of a lot cheaper to get involved than it was in the 80's and beyond, so it was a case of the best that made it, not the just the ones that could afford to continue. I know how much it costs to race Karts at club level, and it ain't cheap, it broke me very quickly.