Daredevil #1, "The Origin of Daredevil"
The basic building blocks for the Man Without Fear are laid bare here. A young Matt Murdock loses his eyesight saving a blind man from oncoming traffic after being exposed to radioactive waste. The loss of vision comes with supernatural enhancement of his other senses, which he hones into becoming a vigilante fighter whilst also completing law school at Columbia. After his father, Battlin' Jack, is killed by the Fixer for failing to take a dive during a boxing match, Matt becomes the costumed vigilante known as Daredevil who seeks justice for his father. Matt also opens his legal practice with his friend Foggy Nelson and paralegal/future love interest, Karen Page.
A dense issue that pretty much sets the stage for decades of stories to come makes it an iconic feature. Though Daredevil's origin would be modernized many times (most notably by Frank Miller's and John Romita Jr.'s The Man Without Fear miniseries and Jeph Loeb's and Tim Sale's Daredevil: Yellow), the initiating events largely remains the same. The storytelling here is a fair bit clunky, as were many of Stan Lee's earliest ventures, but it's hard to deny the iconic stature of this first issue on the grander landscape of Marvel Comics. Bill Everett's design for the character remains iconic, even if it needed a slight revamp courtesy of the one and only Wally Wood.