Based on my experiences;

The best:-

BA, Galleries, LHR, T5
CX, The Wing and the Cabin, HKG (prefer both to the Pier).
IB, Velazquez Lounge, MAD, T4
IB, The Dali Lounge, MAD, T4S
JL, The Sakura Lounge, NRT, T2
QF, The Qantas Club, MEL

Not as good as the lounges above, but worth a mention:-

RJ, the Crowne Club, AMM - was pleasantly surprised with this lounge.

The worst:-

MA, The Sky Club, BUD both T2A and T2B locations.

Not surprisingly, most of the best lounges I've visited are at the airline's main hub.

FTers, what are the best/worst OW lounges you've been to? Please do not include third party lounges, only lounges operated by OW airlines.