Easily this one:http://www.amazon.com/gp/product/B001ID4ZY0/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1

I found the glass ones were not sturdy enough and therefore lasted a much shorter amount of time, this ones made out of steel and is very reasonably priced.

I would however point out that letting someone know where you are is more valuable then any compass ;).