As an (almost) auslander - not having grown up in the East - seems to me there are a few locomotives that are uniquely associated with the PRR, so using that as a definition of iconic, here's my contribution:
GG1 - hard to see how this isn't near the top of everyone's list. As someone else has already noted - it was 'so good' it outlived PRR and PC, too, for that matter. I did ride Pennsy passenger trains between Wilmington, DE and Trenton, NJ in the summer of 1966, all pulled by GG1s
T1 - while one could argue about the underlying engineering, there's little question that the Raymond Lowey design was uniquely identifiable as PRR
Baldwin Shark(nose) - while several railroads had sharks, the first picture of a shark that I saw was the PRR version with the two radio antenna 'rails' along the top. This design was also Lowey, apparently inspired by the T1 design.
My last two suggestions are based on model trains. The first is the K4 Pacific - the first electric train I was given (Xmas present) was the American Flyer K5 Pacific (#312) plus the green New Haven passenger cars (nevermind that the Pennsy K4 wouldn't have actually pulled NH passenger cars -- a detail that escaped me as a six-year old)
S2 - the kid down the street had a Lionel S2 engine. We both noticed, when comparing his engine with my K5, that there were no steam cylinders and pistons on the S2, but I don't think we were 'hip' to the fact that the prototype was actually a steam turbine. All we knew was that his Lionel trains had three rails and my Flyer had two - a source of endless arguments...!
One certainly can't argue that he 'Standard Railroad of the World' wasn't afraid to take engineering design risks - along with the T1 and S2, there were the S1 and Q1 (both Lowey designs), and the Q2
Note added: caught by the ol' double negative. the above should read that "one can argue that PRR wasn't afraid to take...risks". Also, just to be clear, as I suspect most of you know, Lowey wasn't the engineering designer, but the industrial designer responsible for streamlining and styling of the locomotives. He certainly had an illustrious career in that era, doing work for Studebaker and even the first jet-powered version of Air Force one (the 707 introduced during JFK's presidency).