So, the idea is to find the Kingdom card that is ignorable (by ignorable I mean you will not buy it) in the least number of Kingdoms. And maybe some close second or third.
My money right now is on Masquerade. Junkers are often really strong, but Masquerade alone often makes them ignorable, so they are easier to ignore than other less powerful cards. I think KC, Goons and Rebuild are hard to ignore, but not even close to Masquerade. The only Kingdoms I imagine to ignore Masquerade are rushes or slogs with gainers (basically Ironwors or Workshop + Gardens or Silk Roads) without enough power to contest the rush/slog with an engine.
Another strange candidate I have is Candlestick Maker, although I do not have the experience to really assert something like that.