Hi,
My primary pen carry is the Fisher 0 Gravity/Police Pro pen in black, it is a clicker, slightly trianglar in shape, has a pocket clip and a rubberized exterior. After about a a year of daily use the rubber wears off in patches but I prefer the thicker, trianglar barrel shape. Here is a link to this pen on the Fisher website,
http://www.spacepen.com/Public/Products/PoliceandFirePen/index.cfm?productID=177I drove over one of these pens with my truck once and crushed it some; a little straightening with the Leatherman, some GOOP glue and it was back on the job.
My back-up Fisher pen is the smaller, thinner Military one that I keep in a spare pocket. Like I said before and LED seconded the Fisher refills run out without warning so you need to carry a spare.
I have also put Fisher refills in cheap Dollar Store clicker pens and used them for a while. It does not hurt nearly as much to drop a $6.00 Dollar Store/Fisher Refill pen in the lake as it does an expensive Fisher Pen.
A Fisher refill is also a good core to wrap duct tape around for multiple use in a PSK.
Mike