I guess it depends what you mean by the "best".
Certainly in the immediate post-War period, the crowds were enormous and speedway was getting extensive coverage in the press. This is also true for the early days in the late 20s and early 30s. There were at least three sets of speedway rider cigarette cards issued and the top riders also found their way in to sets of "leading sportsmen" and the like. There were also hundreds of postcards of speedway riders. I don't think speedway had that sort of following and publicity even in the 1965-1980 period.
Being the age I am I have lots of acquaintances who went to speedway in the 1946 - 52 era, but never went again, not even in the 1965-80 period. When, as I frequently do (!), talk to them about speedway they will remember names like Vic Duggan, Jack Parker, Tommy Price and many others from the immediate post War period. Some will mention hearing of Ivan Mauger and possibly Barry Briggs, but that's about as far as it ever goes.
I would say that speedway's greatest era was 1946-52.