In case you haven't seen the linked videos by GeoWizard about crossing Wales and Norway in a straight line, I would highly recommend watching them. The whole series makes for some very entertaining evenings, you wouldn't believe the kinds of things he comes across on his journeys!
What counts as across? In New Zealand, following Auckland's Portage Road would pretty much do it: <a href="https://www.google.com/maps/@-36.9356481,174.8371974,440m/data=!3m1!1e3" rel="nofollow">https://www.google.com/maps/@-36.9356481,174.8371974,440m/da...</a> (zoom out to see why)<p>Australia though? I suppose you could count the northern tip of Queensland, but it doesn't seem very "across".
Should be ‘Crossing an entire country on a geodesic’<p><a href="https://en.m.wikipedia.org/wiki/Geodesic" rel="nofollow">https://en.m.wikipedia.org/wiki/Geodesic</a>
Unfortunately, the map tiles from openstreetmap are not loading because the API key is being rate limited.<p>It would be cool for the OP to upload his line to this site and see some other metrics: <a href="https://www.scoremyline.com/" rel="nofollow">https://www.scoremyline.com/</a>
If one really wants to stick to a strait line, Monaco is probably not the best. It is certainly small, but it is full of obstacles. For pure straightness, somewhere like Canada, Greenland or the UAE would probably allow you to walk a more-perfect strait line from one coast to another through open territory. Such a trek would be far more difficult but much more strait.
I would be interested to know which is the longest distance straight line crossing of a country I could do, without having to go through a building or having to swim.<p>Maybe some country in North Africa, that is mainly desert?
If this is just about minimizing crossing a country along a straight line then surely you could just pick any 3-country intersection and just cut the corner as closely as possible.