Huntsman Cheese. It is not one cheese, but two in striped layers: Double Gloucester and Blue Stilton. I love Stilton, but it's pretty intense, and the combination with Double Gloucester mellows it down considerably. In the U.S., it's available in any reasonable grocery store with a good cheese section.
From where I'm around, the best Philly cheese-steak is from a pizza place called Bambino's. I could be wrong but I have developed a taste for Philly cheese-steaks recently.