My very first cheesesteaks were in Philly, but out in the 'burbs - in King of Prussia, to be exact - at a small defunct chain called, simply, Philly Steaks. There were two locations I frequented whenever I was visiting an office we had there for several years, and on my very first cheesesteak there, I was hooked. I was then taken to two other places that are known far and wide: Jim's Steaks on South St. in downtown Philly, and one whose name I'll have to look up, in Manayunk, a neighborhood that's located about halfway out to the King of Prussia/Valley Forge area.
All of these are at the top of the list. There was another one called - I think - Pat's, which in the late 1980s opened a store in Hollywood in the Sunset/La Brea area. The exact location escapes me, and I don't have any idea how long it lasted there, but the couple of times I had one, it was absolutely as good as the originals.
I tend to not trust any other cheesesteaks, anywhere, however much they're hyped as being original Philly style or whatever. I can't even remember the last time I've had one. But there is NOTHING like the real thing, fresh off the griddle, with a sprinkle of the hot and sweet peppers that, at least in those days, were customarily kept in large jars, at the ready, for customers to dip into. Mushrooms are fine, too, but you must have those hot and sweet peppers.