As an American Express Platinum cardmember lovely: The Air France lounge at SFO has nice bubbles and noms, the lounges at Boston are nice albeit with that weird blue light, and the last time I visited JFK on B6 the Airspace lounge was just decent, although I've heard someone else is taking it over. The Escape lounges in OAK and RNO are just swell, too. AS does not offer OAK or RNO JFKs afaik, and B6 does.

As a counterpose: try getting into the AS lounge at JFK with Priority Pass, the AS terminal in Boston lacks a lounge (although there is a strange Priority Pass-connected dive bar just before security) and there is the whole debacle about how AS forgot to renew their lease or something at SFO, but at some point in the future we'll have one again.