Which geographic coordinate system /projection would you recommend to measure the length of hundreds of lines which are globally distributed (southern and northern hemisphere)? The lines have a length up to 200km.
If you are going to measure the planimetric length of the lines (
length( $geometry)), I can not recommend any reference system. The deformation produced in the distances when transforming a curved surface into a flat one, can only be worse in some systems than in others, for the points that are far from the center of the projection.
On the other hand, if you are going to measure the ellipsoidal length of the lines (
$length), all the reference systems are equally good and the result obtained after projecting the coordinates of the vertices to any of them will be the same. All you need is to choose the right ellipsoid in the properties of the project.