Another vote for knife, and I'll add it should be a big one. Chop and split wood, hammer with the pommel, make shelter, make other tools, split plant fibers for rope, make a staff, tie it to a staff for a spear, kill and dress game... I dunno, I just feel like without a knife I'd be screwed and with one I could do anything.
mono filament fishing line = sutures, trip wires, fishing, garrote, dental floss, sewing clothing, snares cloths line, and more that I can't think of right now.
No thanks, I'll stick with a knife or multi-tool. I just got back from a back country hike in Death Valley and the item I turned to most often was my multi-tool for the knife, saw, screw driver head, and pliers (listed in order of greatest need to lesser need). If you want to allow me two items then give me a multi-tool and a Buck knife.
The idea is to think of items that could be used multiple ways and think outside the box. A knife is a valid survival tool. Knowone is going to argue with anyone who says its the most important tool. Making fire easily is #2. Purifying water is right up there with fire. What's next on the list that has the most uses? What's outside the box?
Since you asked what item in your pack, I'll go ahead and disqualify knowledge, hands, and brain since I have no plans on being dismembered or lobotomized anytime soon. While I personally carry it in my pocket, I could put it in my pack and often carry a spare or two in there, so I'll go with knife as most have already said. While I can technically do without it, it is the one thing I would want most in any survival situation since you can solve most any problem with it given the right natural resources around you.
normal strike on box matches (powdered heads for propellant) and light anywere matches (tips flaked mixed with water or saliva, or even blood and pressed into old primer) can be used with a pair of needle nosesd pliers (crimping in bullet and re-forming deformed bullets) and a small nail (cleaning out spent primer with sharp end, reseating with flat end) presto field expediant reloading kit.
this is corrosive as shit, so be sure you are near fresh water. low pressure grain for grain with modern smokeless powders. not somethig i'd pack int he BOB or even recomend doing - but somethign to keep in mind for that one impossible situation you find yourself in.
A spoon. Here are some of the things you can do with a spoon.
eat
cauterize wounds
use as tent peg
use as toggle
sharpen into a knife
crush herbs and medicinal plants for poltices
make an awl with the handle
stir things
and many more.