Keeping it within oneworld only:

Best - QR CDG (yes, my favourite is not even technically an F lounge!). Very honourable mentions go to QR LHR, QF MEL F, QF SYD F, Al Safwa, The Pier F HKG and CX LHR F. I think if you have Concorde Dining BA IAD is the best of BA and is a great lounge in the grand scheme of things, without Concorde Dining it's nowhere near any of these, though.

Worst - Some of the complete dumps passengers are sent to at various mainland Chinese airports, I have literally chosen to sit in the main terminal before instead.