I want to thin slice and pan fry beef, what cut?

A Lot of it is how thin and How you are slicing the meat. Hand slicing will obviously not be as thin as a meat slicer. If a meat slicer, pretty much any cut. If not, flank or a rump roast will do. 

I've use blade in the past. Rump is traditional. I always slice my own.

I use top round, ideally London Broil. I roast it off to 128° or sous vide for three hours at 128.5°. Sometimes it's served as steak, then the rest of the cut goes on the slicer to make really thin strips. This will cook, and be tender, while the stir fry veggies are being cooked. This does not cook up crisp and fatty. It's a fairly lean cut, and I don't care for beef that has been cooked until crisp. But it makes a great stir fry.

The mention of London Broil is because the width and depth of the cut are good for making bite size thin strips without further carving.

If you are using it for stir fry strips, a piece of 'flank' / strip steak would suit, it slices into strips well. 

In most grocery store's meat section they offer meats specifically for stir frying. They are usually already thinly sliced.

Loin would work. Almost any steak would also work.