Chevrolet S-10/GMC S-15
You might not consider the lowly S-10/S-15 one of the greatest GM cars ever, but it was. These were the first mainstream compact pickup trucks released by GM that didn’t share an existing Isuzu platform. The S-10 also brought the Blazer to the market, which became a competitor for the Jeep Cherokee (via Car Gurus).
These trucks and the accompanied SUV were the birth of the modern sport utility era that we have today. The S-10 was one of the longest-running vehicles in production at General Motors, and it ushered in a new era for the automaker.