I voted for 1967-8. Between the '67 Shelby G.T.500 and GT Fastback, I don't think there was a better model year, and the '68 KR Convertible is my favorite car of all time.
The 1969-70 era is second in my book. I love the 1969 Mach 1 and 1970 Boss 302, but I don't like the Shelbys from '69-70...