Well, I definitely share your love for Dennis O'Neil's work. But I do feel there's quite a few contenders in that category, including Roy Thomas, Len Wein, Marv Wolfman, Doug Moench, Steve Englehart, Steve Gerber, Don McGregor, and Alan Moore.

And also names I have a degree of contempt for, whose work doesn't speak to me, such as Neil Gaiman, Garth Ennis, Warren Ellis, and Grant Morrison.
It could be argued if not for O'Neil, a lot of these other writers would not have followed down the path O'Neil blazed. Or be working in the field at all. I still wonder how much of the writing on Miller's DAREDEVIL run can be credited to O'Neil's guidance, and perhaps uncredited plotting. O"Neil has said in modesty that it was all Miller. I'd love to ask Miller's account of that if I met him at a convention.