My 1971 Kawasaki 500 H1 (Mach III) was the most fun. I bought it new in 1972 as a senior in high school. There was nothing that could beat it, car or motorcycle, from 0-100 mph. Now, it had no bottom end power whatsoever, but, at about 5800 rpm that screaming two stroke came to life. It put out enough smoke to kill every mosquito within a 1/2 mile radius and had a rather nasty "twitch" in corners but the fun factor was unbelievable. The only way I was able to keep my driver's license was that the local small town police were rather forgiving in those days. I can't count the number of times I was stopped for my "7,000 rpm burnouts" which explains why the original rear tire lasted less than a 1000 miles. I was also told by one forgiving officer that I was driving a motorcycle, not a unicycle, after being stopped for one of my famous 150 foot long "wheelies".
I was the king for about 4 months until a friend of mine had the audacity to buy a 1972 Kawasaki Mach 750 Mach IV. My reign was over, I was moved to second place. I could stay with him all the way through first gear and a little through second, but then it was over. I could still beat every Honda, Harley, Norton that tried but the bigger two stroke just ruled the day.
So for me, the totally unrefined Mach III was clearly the most fun.