there are several that could fit the bill but i'm not sure which one is the longest having been off the 320 for so long now. YYZ-SJO,
JFK-YVR, YYZ-MEX, YYZ-BDA(Barbados), YYZ- Caracas. These are all ove 5 hrs. For years there has been talk of doing transatlantic with 319s in all business class config but it's only talk for now.