If there were to be a King...IMO there'd be one in each brand, and although the LS6 Chevelle would be held in high regard, it would not be king among my GM pick. My order might be something like;
Chevrolet = ZL1 (undoubtedly King)
Mopar = Hemi Cuda/Challenger (draw perhaps)
Ford = GT500 Shelby
I'm sure we could go into others lines like Buick, Pontiac etc. where each lineage holds it's own "King" but I'll leave that for another day as the list would no doubt be long.
There are a few collectors around who hold each of these in their collection.