I'd have no issues with someone saying Roberts, but I think the level of competition was so great during the late 70's to the early 90's that each rider took away from the others, that picking one rider is near impossible , Sheene, Roberts, Spencer, Lawson, Rainey, Schwantz. and throw in Mamola as the Joker. and then who do who go with Roberts, Lawson, or Rainnyyesterday I nominated Mike Hailwood and Kenny Roberts
(then deleted the post as undignified and unproductive, a statement of the obvious)
Lastly I can't forget that Ago raced the 2 class at the same time, As a former bike racer I can tell you that's not the easiest feat to do, no matter the level of machinery or competition faced. You make a mistake, You go down.