Is it Singapore Airlines or Qantas that is currently holding the record for the world longest non-stop commercial flight?
I assume you are referring to the recent Qantas flight from New York to Sydney:
Australian carrier Qantas has completed a test of the longest non-stop commercial passenger flight as part of research on how the journey could affect pilots, crew and passengers.
The Boeing 787-9 with 49 people on board took 19 hours and 16 minutes to fly from New York to Sydney, a 16,200-km (10,066-mile) route.
This flight was primarily a test flight to study impacts of ultra-long haul flights on passengers:
Qantas has announced three ultra long-haul research flights to gather new data about inflight passenger and crew health and wellbeing.
Because it was just a test flight, this does not count as regular service, which would not make sense with the current aircraft anyway:
No commercial aircraft yet has the range to fly such an ultra-long haul route with a full passenger and cargo load, Reuters news agency reports.
To give the plane sufficient fuel range to avoid re-fuelling, the Qantas flight took off with maximum fuel, restricted baggage load and no cargo.
The current record for longest regularly scheduled flight is indeed Singapore Airlines flight SQ21 between Newark and Singapore according to the Wikipedia List of longest non-stop flights (retrieved 2 Nov 2019, based on great circle distance, thanks to Michael Seifert for pointing this out).
The record for the longest ever flight with a commercial airliner however is still held by a Boeing 777-200LR:
In a trip that included two sunrises, a new long-range Boeing 777 flew more than halfway around the world nonstop on Thursday, breaking an aviation distance record set by a B-52 bomber 43 years ago.
This flight was primarily a publicity stunt resulting in a Guinness World Record. It was never meant to be used commercially since they intentionally flew a longer route.
It is difficult to determine the longest actual flight route as, depending on the route chosen by the pilots, the distance varies. The route from Qantas, JFK-SYD measures 16013 kilometers. This is roughly 600 kilometers longer than the route SIN-EWR operated by Singapore Airlines.
Looking at the data provided by FlightAware for this flight operated by Singapore Airlines, it can be seen that the flight from October 31st, has flown 16850 kilometers. Thus a longer route than the scheduled route by Qantas.
In general the distances for the longest route are determined using the great circle distance measurement. Then the route flown by Qantas is the longest route, which in practice will be actual longer than the 16013 kilometers