Putting the Pieces Together, by far!!! No other card in Doomtown has such a fun mix of risk/reward, as well as entirely dictating the draw structure of your deck. A well oiled PTPT deck that fully comes together during a game is like watching an Olympic athlete - it seems so easy (and if you're against it, possibly unfair), but you have no idea what work and sacrifices were made to get to that point.
Decimator Array is a close 2nd, but you can ignore the hearts straight flush portion of it, and still enjoy the massive benefits of an influence, minus upkeep, and sidekick. Technically it can sometimes be useful in normal 3 value decks, too, to either avoid cheating or turn your off value hearts into an on value card.