There are many thoughts on this I'm sure, being a sportbike forum. Most here I suspect are younger than I, and as such may be 'inhibited' by knowledge on certain milestones in performance themed bikes such as:
1]First motorbike to reach 100mph.
2] First motorbike to actually be called a 'Superbike" in popular advertising.
3] First motorbike to be specifically offered for street use for specific purpose to be allowed to run in WSB and other world endurance racing.
There are other milestones that superbikes have achieved, and hope to hear more.
My personal thoughts are, that 60 yrs before the word 'superbike' was even used, there was a streetbike that dominated all vehicles on earth, and to this day is probably the most collectable motorcycle on earth. the Brough Superior SS100 Alpine Sports.