Depends on gates and timings. The BA lounge is by far the best, but otherwise there are 3 US Club lounges and one AA lounge. I would go to the US Club nearest to your departure area, PHL is a bit spread out with long wings off a main corridor. The one between B and C is the biggest I think. The reason being that if there are irrops or other problems - and there frequently are such issues - the US Club is best placed to assist. As a GCH all are open to you.