There are a lot of people that swear by the Amaroso brand hoagie (sub) roll. I find that most cheesesteak places in Philly do not use that brand, but it is very close and commercially available so it is the go to roll. Basically, it is an Italian white hoagie roll that has a softer crust.
From Original Philly Cheesesteak 101:
"The most common characteristic of a cheesesteak roll is that it is light, crisp but not overly crunchy on the outside and soft, tender but not overly chewy on the inside.
Rolls should be long, uniform and slender, but not skinny. The ends should be rounded, and not pointy -- like the wider side of an egg so that the steak fills the roll evenly and every bite will yield the same bread to meat ratio. Some operators will slice off the ends of their rolls to guarantee a perfect ratio."
People say the bread makes it, but I lean toward technique. After cooking and chopping steak and onions on flattop, you arrange it in shape of roll. Then you put cheese on top of that and put the sliced roll over the meat and cheese on the flat top and press down a little so the grease from the meat creates a slight crispness to the edge of the roll. Flip over with a long spatula.