"the" most painfull submission/lock/pin

[mai tai]

there is a choke from the full mount where your left arm goes around the back of the neck and the hand tucks into your right elbow your right hand makes a fist and goes into the side of the neck or throat.

this thing kills. the others hurt but if i tap its over....... this one hurts the nexted day.

[Gunyo Kogusoku]

Ashi Hishigi (the heel hook), Ude Garami (Musha dori/Americana) and Kote hineri (Kind of a similar wrist lock to Aikido's Nikkajo).

[goshinman]

Any hold that puts pressure on the shoulder is very painfulin my opinion. Ankle locks are painful too.