Now, Meglet has been to my house, and has seen a fair percentage of my collection, though by no means all. My immediate response, of course, is, "Only one?"
I'll settle for four:
The stainless steel Case brand Boy Scout knife I bullied my mother into buying for me in 1974, and have carried in my pocket ever since. If, at some future time, you ever want to summon my ghost, it's one of the things you should really have on hand.
The ninjato that talks. I was in a store planning to buy another sword, and picked up this one on a whim, and it said, "Let's go kill something." (I am not the only person this sword talks to, by the way...) You have to buy them when they talk to you...
Two others that I am fond of because they are pretty, and old, and were made with the assumption that they WOULD be used on human beings: The US Army Non-Commissioned Officer's sword, made in Trenton, NJ, in 1863, and the Colt Government Model 1911 (.45 handgun) manufactured in 1913 (a legacy from my Great Uncle Robert).