The '67 Shelby. It has either a 390 or a 428 c.i. V8. They are monsters and were pretty much the most Iconic muscle car ever.
I like the 1969 Camaro, but those are too common with muscle car enthusiasts. They are great but don't come close to a Shelby especially a SS. A Z/28 is a more even match, but the Shelby still out-performs it hands down.
And as for the 1968 Pit Viper, I would suggest that over the Camaro, its a modern muscle car. I love the look of them, and those you never see on the roads. I just Googled it right now and they are great. Very modern yet retro looking (since its on a '68 body and chassis.)...I like it, it looks like the Eleanor from Gone in 60 Seconds.
It all comes down to if you like an all stock (original specs) car like the GT500...or a Custom car (the Pit Viper.)
When i was a kid my uncle had a 1966 camaro SS so i suppose that a 1969 wouldn't look much different. Also if you can get a convertible. I probably only say the camaro because that is one of the coolest lookin old hot rods.