Where am I? What season? In both cold and or dry extremes, the filter would be mostly useless. In hot extremes, a fire becomes much less desired than in cold extremes. If it's cold, is the clothing on my back sufficient to sleep a night in, or will I need to construct a warm shelter?
In a moderate climate, with sufficient clothing to need little extra protection, and sufficient open water in my environment, I know I can survive a week with only the water filter, even if I don't eat. I might not enjoy it, but I'd probably even end up a bit trimmer than I started.
In any frozen climate, I think I'd vote for the firestarter. Again, for water purposes. I could probably succeed in starting a fire without it, but I can build a shelter without a knife easier than a fire without a firestarter.
In a desert-type environment, I'm torn between the tarp and the knife. Water I get will be from digging or from plant life, and a knife would help me accomplish those tasks. OTOH, a shade shelter might be difficult to create out of natural elements, and the tarp would be helpful.
So, I guess I want all of them. Oh well...
Stryver, back for a bit