I think it's a matter of how the question is interpreted.
If the question is who had the least impressive performance (comparing these ten playoff runs to each other) - the answer is almost certainly Crosby in 2016.
If the question is who was least deserving (looking at how deserving these players were of the Conn Smythe, comparing them to all of the other players in each year) - the answer is probably Williams. I think he was less valuable to the Kings than Kopitar, and probably Doughty as well. The 2016 Penguins were a balanced team and I don't think anybody was clearly more valuable than Crosby (Kessel arguably was, but it's not clear-cut, as it is in the case of Kopitar over Williams).