Detective Comics #38. The first appearance of Robin and the comic where Batman became Batman.
Before Robin, Batman was a second string character, like Green Lantern or The Flash, having enough draw to be on covers, but not enough for his own book. Robin changed that over-night. Sales of Detective Comics doubled after Robin was introduced and Batman got his own title two months after that introduction.
That's not even mentioning the creative impact it had on the character. Having someone for Batman to talk to improved the writing dramatically.