Agree on all points.Redlines are redlines. I wouldn't call those collector true collectors
I can see some trying to break the redline era down as "Spectraflame Era", but there are exceptions to that rule. Do they also consider the Jack Rabbit special, Pit Crew Car, Snake, and Mongoose not real redlines? What about the enamel versions of the Mark IV and the Heavy Weight Semi cabs?
As far as I knew anything that had red stripes on the tires and was made from '68-77' would be considered a redline.
As far as the rarest one,hard to say.As fas as a "holy grail",I would say that is based on what you like and what you want.Just because something is rare doesn't make it a holy grail.
Superfine Turbine off the top of my head would be one of the least used castings ever.One year if I remember right.
I have a holy grail or two myself as far as my die cast collecting.The few that are hotwheels are cars were made within the last 15 or so years.
My main HW grail is the Legends Cadzilla set.It was way to much when it came out,and I missed out when they were blowing them out at K B Toys.