I am not sure if I own it or it owns me but my brain and sanity might be a good answer. Perhaps I might omit the sanity part because it just comes and goes as it pleases.
Computer - It would be near impossible to find what you want to buy without one, even a light bulb can be a tricky business these days, so many to choose from and never the one you need
Most useful for me is a toss-up. Without my 'reader' glasses, I can't make out anything up close. Can't read anything. Can't see to work on anything. Second is my hearing aids. Without them my music is lost. Conversations just can't happen. So they team up. They keep me happy and able to get though my day.
Just about everything I own is useful.... I could be here all day listing everything that most people have too What about you Mr Pasty ?
First, I guess, my glasses. I can see without them but can see much better with them. Then, my car, for obvious reasons.
My thermos bottle. It keeps the hot things hot and the cold things cold and I can't figure out how it knows the difference!!!