Fiora's R
I feel like it's way too easy to get away from this in anything other than a 1v1. This seems counter-intuitive considering the huge team fight benefit it has. Basically, I ult someone, get slowed, stunned, etc., target walks away. This happens fairly consistently. It really only works well when you can get someone trapped or grabbed (i.e. Blitz) but pick-off kills don't take advantage of the heal the same way a mid-teamfight kill would. I don't have any solutions for this - just thinking out loud.