I normally carry a nice standard Fisher space pen in tactical black, nice slim solid pen but have started to use these little goodies http://www.heinnie.com/Pens/Tombow-Pens/c-1-290-768/ Tombow Airpress and Tombow Pfit, while not heavy duty they're very light weight, compact and come in funky colours, maybe one to match or contrast with a SAK The Airpress pen will right upside down like the Fisher space pen and the little Pfit has a great pocket clip. Both very cheap, even the refills, worth a look
I quite like the parker jotter with a gel ink insert, the gel ink dries almost instantly which is a huge improvement on all other gel pens I've tried.
A Bic. Sometimes you have to go quick and dirty.
I like Parkers,they just never last longer than 30 seconds