‘65 Mustang fastback. Not even a contest.
Had one I modified to run low 13’s. Too slow. Bought a ‘69 SS 396 Camaro, dropped in a LS6 454 and ran mid 11’s with 9” slicks, 4:56 gears. This is my candidate!
Yes, you nailed it. The original fastback Mustang was the peak of this genre.
Sorry, but the Mustang, Camaro, Challenger and ‘cuda ARE NOT MUSCLE CARS.
They are all PONY cars. A muscle car is BY DEFINITION a mid-size sedan with a large displacement engine from a luxury/full size sedan.
Pony car, not a muscle car. Fast, but different category.

Ultimately, the quality was horrendous and traded-in for a new 69 VW bug at BobSmith's Hollywood VW.
The showroom floor featured a Porche 911...retail sticker was $5,000.
Technically a mustang is a “pony car”, not a “muscle car”.
While they overlap, they are separate categories.
Pony cars are lighter and top out at 3500Lbs. Handle better. The muscle car are all about straight line, off the line, acceleration and were generally based on a mid sized sedan.