Neck, wrist, elbow. I'd say those are the weaker.
The easier way to decap is an attack to the neck, it doesn't take much strength.
Wrists break many times just by pressure, pressure after being damaged or hitting the ground pretty fast/hitting the ground if damaged.
Judo is a lot about wrist breaks.
Elbows are also a weak joint, a kick can break it without much strength, and they'll break quite easily if bend to the wrong side. Even though they are a bit stronger that the previous ones, they're still quite weak.
But as the most weak of them all, I'd say wrists because not only are they weak, they're also exposed to a lot of pressure from the grabs.