I think the reason for so many responses with firearms at the top has to do with the nature of the scenario put to us. Since most of us would prefer to shelter in place in more mundane survival circumstances, trying to think of things for which I would want/need to bug out for has me thinking that the situation has to be fairly catastrophically bad. People are going to be quite desperate and the threat level is going to be relatively high (there's that pesky human nature thing again). Given that implied condition, and the fact that I don't mind becoming predatory in such extreme conditions, and that my proficiency with firearms is, well, let's say above average, a gun is the one survival tool that is hardest to replace or even find a reasonable substitute for, and yet makes acquiring the other necessities along the way significantly easier(although a bullet hole through a tent or a coat does make it a little less effective without mending).
This is not a pleasant notion, but it is a realistic one. At least I've seen it proven out firsthand. Here's my way of thinking, for anyone that didn't have a firearm at the top of their list (assuming you know how to use one and are willing to), don't expect to keep anything else on your list for very long if you encounter someone like me out there and I decide I need it more than you do. Them's just the fax.
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The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)