I would think it must be the Camaro and in particular the Z-28 if we are talking muscle only.
However, if Frank really means "significant" Chevrolet performance car and not just muscle car, it must be the Corvette, I'm afraid. It was (is?) the only true American sports car. Between the early Duntov specials, the show cars, the Grand Sport program, the 67 L-88s and the more recent C5R/C6R endurance racing program it has been the only car to really put the US on a world stage. I think that the C5R/C6R program in particular distinguishes the Vette due to the direct factory involvement, the dominance it held over its class and the fact that the road ZO6 was built in the same department. It continues in production form to be "the dogs bollocks", as they say over here.
As for the other marques:
- Ford = the Mustang in all it's various guises
- Pontiac = GTO
- Buick = GSX version of the Skylark
- Olds = W-30 (although I prefer W-31s)
- Plymouth = 'Cuda, in particular the Hemi cars (although I'm partial to the Wing cars)
- Dodge = dare I say the Viper?
- Chrysler = the 300 maybe?
- AMC = AMX
