I think it's the first game that really transcended what games were at the time. Prior to PCs, games were mostly just hand-eye coordination checks. Myst was the first game I played that really engaged the brain and problem solving.
Not sure what you call a PC, but there are 17 years of problem-solving computer games (text adventures, click-and-point adventures, RPGs, real-time and round-based strategy games,...) before Myst.