After a lot of thought, I’d have to say either FANTASTIC FOUR #1 or AVENGERS #4. Although both came out before I was born, I was fortunate (and blessed) enough to receive the later-issued “Golden Book And Record Set” for each as gifts as a tyke from my wonderful parents. Each set came with an exact reprint (minus the ads and cover price) of the comic plus a big LP “record” (a black vinyl disk that people used to play in the 20th century before CDs and digital music) on which voice actors read each part, complete with sound effects and bombastic page announcements. I still recall, clear as day, the stentorian announcer’s tone and reverb effects, along with various actors’ voices and scenes.
I’ll pick the AVENGERS as my single piece, simply because Captain America became my favorite superhero and the heroic story of sacrifice, respect and teamwork (along with that classic cover) impacted me more. The combination of powerful-as-a-locomotive Jack Kirby artwork and storytelling, Stan Lee’s dialog and (melo)dramatic writing and the bold primary colors just transported me to a world of imagination I didn’t know existed and never, ever wanted to leave. I read it till the covers came off. It took me a while to really comprehend that ordinary people created these and that they weren’t magical windows into better-than-reality realms.
That comic book (along with the Batman TV show) is more responsible than anything else for my lifelong love of comics and enthusiasm for panelogogy (comic collecting), as well as my sense of right and wrong, being a hero and unleashing my creativity. It also set me up for a lifetime of disappointment when I leanred that I couldn’t gain super-powers myself! But the trade-off with sparking and setting free my imagination was worth it.