It depends on what you mean by father of rap. I would never consider Dre as oen of the greatest, because he doesn't write his own shit.
I mean Tupac kinda popularized it and made it big. It was already big, but he elevated it to another level. And I think they called Eminem the father of rap because he did the same thing, and got a whole new audience to listen to rap. Some people don't know rap Eminem. It's dumb to call him that though.