Now that the Envoy lounge has been converted into an ordinary US club, the BA lounge is by far the nicest one at PHL, with the new DL lounge a distant second. I have only been there before a BA sleeper service flight, so I don't have a good sense of the ordinary food offerings, but the (self-service) drinks selection is better than UA and DL and well beyond what US offers. It was one of the first of the BA lounges to offer their current lounge design/concept (Galleries) and has a much nicer atmosphere than any of the other clubs.

I don't know if they scale things down earlier in the day when the lounge is occupied by AA fliers rather than BA fliers.