I'm not sure I could specify one item.
Most valuable (monetarily), my home. It also has a very personal value, as much of the design was my own. It is, in that sense, unique.
Most valued, well, depends. I have a number of items that have incredible sentimental value, including some of my mother's jewelry, my father's war medals, and my grandmother's furniture. I can't think of any realistic circumstances under which I could be convinced to part with any of them, and I'd be very upset if they were lost or stolen. Their monetary value, I neither know nor care.
Most prized possession .... that's hard. I have some first editions of books that are either irreplaceable, or close to it. Some are also quite valuable. I also have some works of art that are also rare or irreplaceable. These include works by Renoir, Munch, Cezanne, Matisse, Dali, Whistler and so forth. Note, I'm talking about contemporary prints, and even in some cases signed prints, not original oils. The prints are quite valuable, monetarily, because they are in some cases, literally irreplaceable, but if they were oils, we'd be talking about hundreds of millions. I'm not talking about anything remotely in that league. I do also have some oils, and while they're quite expensive, they are not from artists in the class of the ones mentioned above.
I'm not sure I could pick out a single item as the most valued, though, because many of these items are valued in different ways. How do you compare an irreplaceable (one-off original) piece of art, with a very rare Dickens first edition, with your father's medals? I rather suspect, however, that what would upset me the most if I lost them would be the sentimental family items, but until or unless it happens, I can't be sure.