Mine took me less than a split second to decide. It is not the item that started my collection, nor is it by any means the rarest piece I have (which would probably be my Japanese 30x115 case (incoming), or the 37x145 Ho-204 case, or a different choice from my Japanese collection), and no, it's not even the Kwk 43 APHE round which I VERY painstakingly fully restored by hand.
No. Mine is an item that's close to ubiquitous. It's value at fairs over here is a few Euros at most. It's a German WW2 Znderbchse, as used for e.g. the Zt.Zs./30 nose fuzes.
The reason?
My grandfather was deported (from Holland) to Germany in 1942, and he was put to work in a factory in Bochum-Zelte, and was over there until the end of the war. Upon liberation by US troops, he was so much weakened from the extremely hard labour and harsh conditions that he was not even able to flee the grounds of the factory and had to recuperate strength for almost a year in the south of The Netherlands before he could finally go home (in 1946). For him and my grandmother the war was a topic not to be mentioned. It was so much of a no-no, that nowadays we don't know what it was my grandfather was working on.
There are precious little clues; the only two things he brought back were a self made aluminum match-box protector, inscribed with a date in 1942, and the location Bochum-Zelten, and the aforementioned Znderbchse.
The Znderbuchse is in my collection now, and I'm very proud to own it. Though as an item it is insignificant for collectors, due to it's history I wouldn't trade it for the rarest Japanese item in the world.
Cheers,
Olafo