Nikkyo (and Sankyo to a lesser degree for the same reason).
I have a tough time telling how much tension is in my wrist and I end up waiting too long to tap or go down. Nage has to work harder, sometimes goes too far, and it makes for a lot of pain!
I seem to be able to do Nikkyo fairly well. Sankyo I sometimes have a little more trouble with, depending on the variation. Likely because we don't practice it as much.