most damage? kimura or heel hook.
most pain with least damage, the facelock guillotine (aka 10 fingers choke, also seen as Cecchine's Catch Guillotine). major damage is possible like breaking the neck but you can inflict maximum pain before you inflict damage with this, plus only a moron would squirm once you've got this on so it's all your choice from there.
someguy at work knows i grapple and was talking about moves and how he used to wrestle and such and so i let him go all out in a guillotine on me. he locked it up, cranked and i just postured out. when it was my turn i did the catch guillotine and to this day he's always talking about how that was the most horrible shit of his life. good times.