The worst thing to have lost for me, was my old computer's hard drive. I didn't have it backed up, and I lost my entire data storage.
[but its okay now because I have an external hard drive] I'm smart about my data storage now!
I lose my keys on a daily basis. Usually, it's because my mom moved them. Sometimes, I either leave them in the door or in my truck. It's a good thing I live in the middle of nowhere and don't have to worry about it though.
If there is one thing I hate losing is my USB. It always have my most important stuff such as homework and assignments, (and all of my fandom pictures that I downloaded for future usage and/or references). Luckily, my computer has always had the pleasure to have somehow saved a backup copy of my files so that's always good. I just hate that practically every year I always lose at least one USB throughout the school year.