to add to this, there's an orange "shockwave" every time you hit an enemy's stun spot, so you can always tell if you're hitting one or not.
for daityl swords and axes, as you've noticed, the stun spots are the legs and feet (you can hit them anywhere on the legs, all the way up to the hips).
for pettas vera and nex vera, the stun spots are also the legs and feet. nex aelio is identical to nex vera.
for nogleth, the stun spots are the front legs and the head—hitting the back legs will not bring it down. once it's enraged, the weak spot also counts as part of the head.
varas, waulon, chiacurio, eldi scythe, and bigg frogga have stun spots on their weak points.
oruq's stun spots are the head and wings. the head includes the weak spot, and breaking it open will cause a stun, so most people focus that.
bujin's stun spot is, as noted, the entire body, since there are no separate hitzones.
ard banshee, thunder banser, and crag bear do not seem to have stunned states, as far as i know—probably because they're a different type of creature. however, ard banshee and thunder banser will both "flinch", interrupting their current action, if you do enough damage to their head weakspots.