For me , it's still the 500 , 550,and 650. Made with common cradles all those years so there is an abundance of factory powerplant choices available to those building a 500 or 550.
As for the 650, it encompasses all the improvements made to the 500/550 frame over the years including a little higher horsepower, better suspension = better handling, and electronic ignition. I'm not wild on the electronic ignition as I like the simplicity of points , condensers, reg. and rectifier....
That and the fact that they were produced in the most similar fashion for those years that used parts availability and parts interchangeability really isn't as big of a problem like other short production models.
They were large enough to be chopped, small enough for cafe, beautiful enough to be restored to stock, and had enough aftermarket products available for the masses to customize a little or allot.
But I am also with you on the 450's, beautiful machines I have the 72 model but have always had the hots for the bomber.