It really depends where in PA you're riding. There's a lot of trail diversity. Generally, it's rocky and rooty. You'll see a wide variety of bikes....rigid singlespeeds to full-suspension trail bikes. For me, I think that the ideal "only bike" would be a short-travel (3"-4") 29er...something like a Niner Jet 9 or a Salsa Spearfish.

Do you have a shop in PA that you like? If so, I would ask the staff where they're riding and what type(s) of bikes they're riding for those specific trail systems.