Very diffcult partner
Since it is difficult to get the perfect partner in this world, I would rather have one who resists or tries to act difficult than one who would throw himself on the mat before I even do any technique at all. In fact, both types can be frustrating, but at least the first one is an obstacle or a challenge that you might feel motivated to overcome, by using technique or brutal force, no matter. But I can't think of any way to overcome the latter type. Sometimes I stop the technique in the middle and watch my partner take his "fake" ukemi, but these people are usually shameless and this does not discourage them from doing their thing. This is what I consider a "very difficult partner".