Most fun I ever had on two wheels was my old Kawasaki H1F from 1975. A modified and truly mad 500cc two stroke triple with an engine note like nothing else on earth. That shrieking three cylinder wail was incredible. Mind you so was the fuel consumption, 25mpg! A set of Denco racing exhaust expansion chambers added to the formula. I gave it a port gas flow and advanced the port timing midly and it went like stink! It was quite fun trying to keep the front wheel on the deck whilst cutting through traffic, it was very prone to wheelies. I've ridden faster bikes since, but nothing as crazy!
Kwaka H1.jpg
The thumbnail isn't of mine but mine was not much different apart from the colour.