SFO/ZRH is 5826
LAX/ZRH is 5939
LAX/MXP is 6037
I don't think that Olympic Airways, Turkish Airways, Aeroflot, El Al, or FinnAir have any west coast gateways, so I think that LAX/MXP is going to be the longest scheduled non-stop from the US to Europe.
Of course, if the requirement is just transatlantic from the US, then South Africa is fare game, and it would have the longest flights.