My vote, from a historical context is Romance.
Yes, one can argue that Combat, Western and Horror were more popular.
However, even for as sexist as they seem today, they were the first foray into finding a way to bring girls/women as a demographic into the comics genre.It made comics realize that there was a potential to bring in a major demographic. Had it not been for Romance comics, there would not have been the surge of female superheroine characters once the feminism movement gained traction. That was the opportunity to for female characters to kick in the door. And it took another generation (or two) to take some of those talented fans and have them become key creative talent in comics.
While not the fiscal impact of other genres and even a hiatus until changes in our cultural landscape happened. Romance comics were the initial experiment that showed females could be interested in the comic book medium.
Historically speaking, from those who consume comics in the modern world, female writers and artists, to the entire realm of female heroes and villains that populate pages today. Romance comics paved the path by proving if you could find the right cultural hook, there was an entire fanbase just waiting to be spoken to.