Well I guess that depends at least a bit on your definition of "longest trip". Do you mean longest trip without stopping over for a night? Or just the longest trip one has taken using Amtrak?
If the former, then mine would be riding NY to DC to Chicago to LA using the Capitol Limited and the Southwest Chief, along with a Regional to DC.
If the latter, then that would be riding the Crescent NY to New Orleans, the Sunset to LA, the Coast Starlight to Seattle, the Empire Builder to Chicago, and the Lake Shore Limited back to NY.