Hardest Pool Event?

Upon reflection this depends heavily on how one defines "hard". If hard=painful, then I will stick with my 200 BK & 400 IM comments. I think the same two events come out on top for required skill too. The 400 IM has all the strokes and all those tricky turns. Backstroke is the most technical stroke. BUT...
if "hard" means "difficult to get right" then I think it is a near tie between the 200 FR and the 50 FR. I think I have swum the 200 fr correctly in competition about once in my entire life, and I'm still waiting to get the 50 FR right. Somehow the 100 doesn't seem so tricky.

For this question, hard=painful 

For this question, hard=painful