The best one is the one(s) you own.
There is no "right" cb750, they are all great. that being
said I prefer the 73-76 K3-K6. Which seem the have largest
parts availability and most interchangeable parts
however I would like to own a very early K0 model.
also note that production went on, Honda was enabled
to modify and make improvements, so in fact
every successive year the bike only got better, but the (vintage) SOHC series
pretty much ended at 1978s. Then came the DOHC.
If you are a first time buyer, I would suggest you find a whole
bike, preferably running. The less you have to do
for your first experience, the better your first experience
will be. Doesnt have to look great, but mechanically sound
with little if any engine or electrical repairs.
My first was a 76, all it needed was a good carb cleaning.
It wasnt until later I started getting into rebuilding an engine.
For which I bought a piece of crap engine out of a junkyard to
practice on. It had a giant hole by the sprocket and blew a rod
so I had no issue throwing it on a stand to learn how it all put together.
The practice engine is a great investment if you want to
go deeper into these motorcycles.