This would be a terrific question for the oneworld Forum, as there are several (including JL Sakura, though it can get crowded as it's not overly large and they're apt to turn non-JL flyers away due to crowding, plus it's inconveniently located for BA, and the Cathay Pacific lounges.

That being said, the British Airways Terraces lounge might be the best oneworld lounge. The offerings are quite decent, and they normally save you some steps to boarding. Whereas you ordinarily would take the lift back up to departures, process and then go downstairs to board, the BA lounge offers a "back passage" to the boarding jetway, making it by far the most convenient lounge.

You can find further information in:

The FT BAEC Lounge Dashboard here, and

The oneworld lounge access and guide tool here.