This has been very interesting but I must agree with those that say comparisons are unfair and impossible.My own opinion is that motorcycle racing has divided itself into 2 distinct catagories, Road Racing and Track Racing and although I know this division allways existed the saftey factors have altered considerably.
In the 50s/60s many of the short circuits were pretty hazardous with poor surfaces and little run off as in many cases they were as the military left them at the end of WW2 so riding the road circuits was not such big deal,just a change of scenery.
Regarding the merits of each rider it is down to the era you were living in and as previous contributors have said comparisons cannot be made,what you can say that those who have been mentioned are all excepional riders.
Looking at a whole life in the sport for safe preservation and ability I would say that Phil Read has been an exceptional motorcyclist