I assume its the very dense EGLL-KLAX but I don’t know.
It was confirmed by the staff to be KLAX-KSFO. It’s a short route with very scenic views.
Interesting, I always see EGLL and KLAX full daily in the G2 server, with lots of planes going back and fourth through those airports.
EGLL to KLAX might be the most popular long haul route, but most simmers here aren’t able to fly such long distances with their device so KLAX-KSFO makes the popular choice as said by Mattheus.