What champions do you find BOTH Frustrating and Fun/Fair to deal with?

All champions can be frustrating to deal with, but there are those which almost make you roll your eyes once they get going. I am talking about champions and champion mechanics that can be a bit frustrating to deal with but feel satisfying and fun when dealt with correctly by your teammates. E.g
I don't think anyone who's played league long enough, hasn't been in a game where you thought the team fight was over after you killed 2 people on the enemy team the rest retreated and you pressed for an objective, then all of a sudden a wild appears and proceeds to clean up your low health team that has blow all it's cc on the ADC and katarina mid. While frustrating to deal with a Yi if you've spent your cc he's pretty fair in the fact that all you require is to burst him down immediately or cc him and it's over and it's always satisfying when your team deals with him correctly.