Where is the wing sweep angle measured from?
Taking the 747 example you mention, on Wikipedia the wing sweep is stated as 37.5°. Three references are given, checking one of them reveals that measurement was taken at 1/4 chord.

As you have noted, there are many angles, and in aircraft design they are all relevant to the designer.

The 'wing design' chapter from Hamburg Open Online University states:


  The sweep angle $ϕ$ depends on the % line on which it is measured. Normally the sweep angle of the leading edge $ϕ_{LE}$, trailing edge $ϕ_{TE}$, 25% line $ϕ_{25}$ (quarter chord sweep) and 50% line $ϕ_{50}$ are stated.


So normally in design 4 angles will be stated. Otherwise, e.g., when checking Wikipedia, the references used should typically be clear. Below shows two $ϕ_{25}$ angles, for example: