What is the rarest LEGO production element (colour/shape combination)?
I would tend to say that any part produced for only one set would qualify, especially if said set is priced high. Looking at the inventory of 10179 Millenium Falcon UCS for example, the following candidates come to mind:


Light Bluish Gray Boat Mast Rigging Long 28 x 4
Dish 10 x 10 Inverted (Radar) with Radiating Spokes Pattern on Concave Side although it's printed so I'm not sure you're willing to accept it. I would have mentionned minifigs as well, but only Obi-Wan Kenobi (Old, Light Flesh) is exclusive to the set and its individual parts are used in other sets.


Considering the price the set goes for nowadays, and the fact that most people who buy it will probably build it once and leave it so (that is, if they open the box at all), I think the grey boat mast rigging is a very good candidate. There are four people selling some on BrickLink as of february 2013, but a glance at the prices asked comforts me in my reasoning. Even the printed radar dish is cheaper.