For hook knives, make sure you have at least one.  In the event you need one, you want one that works.  Replaceable razor blade style knives have been known to fail on the first cut.  Although the razor type would most likely work, we use only the sharpened steel blade style (see below) knives to mitigate as much risk as possible.  When picking knives, keep in mind the mounts and placement.  Make sure their position and mounts don't interfere with other gear choices or your reserve and cutaway handles.