Schumacher was genuinely brilliant in his first stint in F1, what brings him down a notch in some eyes (and I admit mine as well) was the early 2000s and Ferrari's attempt to trample the sport under their own machinations. They had bespoke tyres made just for them and seemed to get so many rules modified to suit them the F in FIA was alleged to stand for Ferrari. There were no testing restrictions in those days and Ferrari had their own F1 standard track at home base, he'd literally run hundreds of thousands of miles over a season as they didn't care how much money they spent, it was worth it to trample the opposition. They would probably have won without all that crap but they decided that's how they wanted to play the game and that's that. It has literally taken until now and Alonso being one of the nicest drivers in the pitlane (and that's a turnaround story in itself) for people to have any respect for Ferrari. I know I wasn't alone in stopping watching F1 in the early 2000s and didn't return until 2008/2009 or so.
Vettel is like a Schu 2.0, he has obvious talent BUT the RBR has been so far ahead of the competition that it has been hard to really compare him to other drivers. Alonso is easily the best driver in F1 atm, he single handedly pulled last years Ferrari (which was basically a shitbox for the first half of the season at least) to within a few points of Vettel by the end of the season. Then you have Hamilton who is more talented than Vettel (imho) but due to McLaren seemingly trying not to win championships for the last few years he only has a single WC to his name. A straight shoot out with modern Hamilton/Alonso would be a sight to behold, shame the one season they were together they were both too young to deal with the pressure and it ended so badly.
Prost and Senna is almost like comparing apples and oranges, their styles were so completely different but they were each so succesful. The year they won 15 out of 16 races between them when in the same team will probably never be repeated and that car is still looked upon with awe. They both won championships when their cars were dominant yes but they both also won races in shitty cars they had absolutely no right to as well. Senna especially could make cars dance round a circuit, even in the worst of conditions.
I can't comment on Stewart as he was before my time and the sport/business has changed so much since then.
As far as the statement about it could be anyone and they'd win race after race, I remember the rather cruel joke in the year that Damon Hill won his championship with Williams that if his 2nd driver had of been a chimp it would have scored 10 podiums that season as the car was so far ahead of everyone else.
I have tried a few times to rank the drivers to finish off this post and I just can't, I can't decide who is better than who. The list of top names during my watching experience would include Schumacher, Alonso, Mansell, Senna, Prost, Vettel, Hamilton and I'd love to say Button but even I can't quite make that statement. If you put a gun to my head I'd have to go with Senna as the best in that era due to him being able to drive anything.