Driving from south Dorset to Edinburgh is the longest in the UK (terrible idea, nobody was speaking to anyone else in the car by the time we got there).
In the US driving from Maine to South Carolina, although that invariably involved a stay at a motel overnight at least once. So wasn't a single continuous car journey usually (maybe once we did it without stopping with my mum and someone else swapping as driver? Can't recall if so...).
Also in Australia we got a coach from somewhere in or near Sydney to Alice Springs (albeit with pit stops along the way at the Blue Mountains and Kings Canyon I think?). Awful experience. We got the plane the way back as I recall.