Nir , With the respect , I strongly belive that Schumacher could compete any of the other three (maybe except Nigel , at the right mood - after all , bigger balls do give you an advantage sometimes
).
Senna is a different story . The only one who (in my opinion) could compete him was villenuve senior. Those two guys never gave up - no matter what , and the hell with the result.
Prost ? I think Alosso reminds me of him. very consistent , quite tallented - but mostly consistent and "cold blooded". The granny - well , I don´t really know how he did it. never looked that good - but stiil - 3 world championships
However - except for Hungary (86 as far as I recall) , he never showed he is capable of going head to head with any of the others.
Shucmacher could definately fight with each one of the three, being almost as fast as Senna , and almost as consistent as Prost.
I trully hope we will have a chance to see Schu and "Il doctore" fighting each other in the same group (Scuderia) next year...