Buick Riviera GS
Buick was not usually a brand that came up in conversations about muscle cars. But there were more than a few muscle cars that Buick released nevertheless. The most notable of which was the Riviera GS. The Rivera GS did not conform to the standard Buick design language, instead, the car was muscular and well adept at cornering and off-the-line performance (via Hemmings).
The Riviera GS shared a platform with the Chevrolet Chevelle and the Oldsmobile Cutlass. The Riviera GS was a car built on the success of the platform, but it fell short in many other areas.