At the moment, iriminage -- I have a stiff ankle that makes the quick, big turn hard to take. I can feel myself stiffening as the turn starts.
Challenging throws as nage, though, seem more situational. Depends on my partner, which version of which throw, whether I am at a seminar trying to get something new or in class teaching kihon waza. If I had to choose, ikkyo and iriminage remain the most challenging to work with overall.