I don't know if there is a "No. 1" item that should be hoarded. Seems to me that a person would want a variety of tradeable items, in other words, don't rely on only one form of currency in a SHTF situation.
Gold, silver, even some base metals will hold their value. Silly as it sounds, I've been hoarding all nickels and pre-1982 pennies for their nickel and copper content. Nickels are 75% copper, 25% nickel; pre-1982 pennies are 95% copper. The zinc pennies go back to the bank. (Yeah, I'm one of those guys that stoops over to pick up small change on the sidewalk.)
Sure, cash is good, unless there is a hyperinflation (Google "Germany, 1923"). Since the U.S. govt took us off the gold standard and removed silver from circulation, Federal Reserve notes are only worth what we believe they are worth. If people get to the point where they believe the paper money is worthless, then they will become worthless. Sort of like today's federal government itself.
Non-perishable food items for personal survival or trade. If forced to choose a "No. 1" item to hoard, food would probably be my choice. And don't discount the importance of clean water for consumption and bathing.
Of course, echoing the sentiments of practically everyone here, I believe plenty of firearms and ammunition are important.
I just bought a crank-operated lantern marked down in price for $15. That joins my crank/battery operated radio that I paid, IIRC, $40 for a few years ago. Common batteries, like AA, D or C could be in short supply after a few weeks/months of SHTF, so these human-powered items might come in really handy. If I had more money, I'd buy more of these gadgets for family/friends.
Anyhow, these are my thoughts.