The best connectors for Low-PIM are those that can be tightly coupled to resist movement.  Bayonet and Snap-in interfaces do not work well in a Low PIM environment.  Small threaded connectors, such as TNCs and SMAs should be avoided if possible, due to the typical power requirements of Low PIM applications.  PIM values are measured in a lab, to isolate and measure the 3rd frequency.  A larger negative value indicates lower-PIM.