DESIRABILITY equates to having pleasing qualities or properties. This could apply to all makes and models to some extent so desirability is actually very individual thing and entirely in the eyes of the beholder.
The most desirable PE for me is the PE175C. In '78 I used to look long and hard at the image below
In '78 I had grand plans to get out of school and get a job to fund a spanking new PE and ride across Australia
it never happened
instead I purchased the RM400C and never looked back
I've 'never' owned a PE but the PE175C/N is the model I always desired. I suppose the closest I ever got would be my TS250..least it's yellow