While the Airbus A350 is technically certified for ETOPS-370, what are currently the highest rated commercial ETOPS flights? Which airline actually operates flights under ETOPS-330 or ETOPS-370 rules today?
Air New Zealand's Auckland (AKL) to Buenos Aires (EZE) is the longest ETOPS flight I know of, under ETOPS 330. It started in 2015:
Air New Zealand completed the first flight approved for 330-minute extended twin-engine operations (ETOPS), flying a 777-200ER powered by Rolls-Royce Trent 800 engines. The December 1 flight departed from Auckland, New Zealand, and landed in the Argentinian capital Buenos Aires about 12 hours later.
As for ETOPS 370, I'm not aware of eny regular commercial flights, but some candidates are Sydney-Johannesburg and Sydney-Santiago de Chile
It was reported last month that Airbus has pitched the A350-900ULR to Qantas, as part of Qantas' project 'Sunrise'. Of the routes that would benefit from ETOPS 370 (if realized) are Sydney to Rio de Janeiro and Cape Town.
Qantas has also earmarked Rio de Janeiro in Brazil (7,312nm) and Cape Town (5,946nm) in South Africa as new frontiers for nonstop service.
The ETOPS 370 region is the darker shade shown below over Antarctica, while the lighter shade is ETOPS 330.