There are probably albums that influenced an entire subgenre of music more than Images & Words influenced progressive metal, but right now I'm at a loss to name one. Images & Words crystallized the prog metal movement that had come into being around the more progressive acts in the metal area and gave it a direction. As a long time fan of the subgenre, from before it even really existed, I can say that the singular power I&W had was this: before it came out, a metal band with progressive leanings was said to sound like Rush, whether it was warranted or not. After I&W, the comparison became Dream Theater. It's second only to SFAM on my DT list, but it is unquestionably the single most important album in the history of progressive metal. While we would have had the genre without it, Images & Words became the blueprint for progressive metal from that point on and progressive metal would be a different sound indeed without it.