Kings and Eat N Park are Western PA chains (although Eat N Park now stretches to Cleveland, MD and VA). What all three have in common is they are open 24 hours and you can get breakfast any time.
King's is a favorite of my wife but I find their food to be a bit lesser quality than Eat N Park and they have the smallest portions of the three. They have a lot of ice cream choices, though.
Denny's loses points with me just because it's a national chain and not local. I do like their hash browns the best, but their sausage is the worst ever. And for whatever reason, the Denny's I have been to are always a bit disheveled. Tables uncleared, bits of debris on the floor.
Eat N Park has a great menu, is the cleanest, and their breakfast buffet is a bargain for sure. They seem to have more cheerful employees as well. They give every kid a free cookie, too. They get my vote.