Technically, Cobras aren't muscle cars. They're sports cars. And if you want to get really picky, Mustangs, Camaros, Cudas, Challengers, etc aren't technically muscle cars either. They're pony cars. Muscle Cars are intermediate or full bodied grocery getters with big a$$ engines.
That said, my vote goes to:
'64 Thunderbolt
or '61 Starliner
