Lost music. It was an underground artist who quit in the mid 2010s before archive culture was really a thing. I've had people dm me looking for it before. I keep it offline only out of respect for the author who wants to stay gone because the reason he quit a decade ago was harassment.

at the same level of importance, every time I've gotten a new computer or operating system since 2013 or so, I've backed up every single user folded (pictures, videos, documents, etc) onto an external hard drive. I have a lot of good memories there that I really do not want to lose.

Early computer aided art and programs I've written, dating back decades.

In the mid 1990s I used ImpulseTracker to create music. The music sucks. But losing the original source .IT files would be heartbreaking.

Likewise, my first programs, written as a child in MS DOS batch files circa 1991 -- basic menu driven interfaces that facilitated launching my installed sharware... I don't have the games the program points to anymore, but that isn't the point ;)

My 3TB of injected VC games for wiiU (And soon additionally about 8 tb of injected VC games for 3ds)

Like most of us, I have plenty of pictures and such that I don't want to lose. The most important to me, though, are some of the documents that I've saved or scanned from years passed. While a tax return from 2003 or the title for a car I owned 20 years ago aren't exactly useful, it's fun to occasionally look at some of the old stuff and see how far I have come in my life since then.

Mostly old projects of mine like artwork and games

Other than family pictures, etc., I've been working on a collection of the episodes of a show from when I was a kid in the late 80s, "Long Ago and Far Away", showing fairy tales from around the world in a variety of art styles and media, with James Earl Jones as host. I've found maybe 2/3 of the episodes so far. Every few years I do another sweep to try to find more.

I have emails going back to YAM on Amiga in 2002, some games I wrote for GBA and mobile platforms that don't exist any more. I have some old IM logs but reading that was so cringe I should delete it, with wipe not rm. Some of the obscure old music I have isn't available even on Soulseek (except when I'm sharing it). Source code of old programs I and my friends wrote. And photos, of course.

I have some copies of shows that only received pilot episodes or shorts such as Wet City that I think are pretty rare. For treasured though I've got a journal from when I was in high school. A lot of it's insignificant forgettable stuff like going to the movies with friends but the prompts of setting help me paint a vivid mental image of it that makes me feel a bit nostalgic.

I'm on an opposite end of the spectrum. I keep almost nothing