Uhm, not sure what kryptonite is, but if it is meant my fav techinques, I can say which are the ones that normally seem to come from my hands more frequently:
1st place: Kotegaeshi
2nd place, at good distance: Sankyo
plus a strange technique, apparently unknown, that I stumbled into by chance a few days ago and that comes to me so natural easy and effctive that it seems to be becoming my fav one: http://www.aikiweb.com/forums/showthread.php?t=19781
PS i read: "Kotegaeshi, I never make a right one". I humbly offer my suggestion, as a bad aikidoka, about what makes my kotegaeshi normally effective (well, I also fell face down twice doing it, as I was doing tenkan also and lost footing :-) ).
With a provision though: you need to succeed in placing your opponent wrist in the right position: our ukes seem to place them already in such position, but a more aggressive uke won't, and you may end up with sort of a nikkyo as you try to get his wrist right for a kotegaeshi.
Given that, his wrist placed with palm upward, if you are placing a kotegaeshi on his, say, right hand, I see many persons focusing almost entirely on their right hand namely the one apparently more involved in the projection. But you have a left hand there too... a kotegaeshi enhances significantly if you use also your left hadn to
1) cut on the wrist
2) plummet it down to touch your hip with it
3) now you use your right hand
Also, do not swing uke's arm around - kotegaeshi on an entirely vertical plane, possibily.
If I misunderstood and kryptonite or whatever that is means the tehcniques that I am not good at, well - all the rest! :-)