For me. I'd say anything that holds any sort of emotional value. If you really think about it, and I do quite a lot, none of the stuff we have is going to really go anywhere except a landfill one day.
I think this all stems from losing my uncle last year unexpectedly and having to help clean out his place. The amount of his stuff that held value you to HIM but not us. It ended up either going into a donation bin or the trash. Bags and bags of stuff just, going to charity for someone else to sort through and who knows if they actually saved it and didn't end up tossing it?
I realize that for a small part of my idol fan loving life, I was using the purchase of goods as a means to making myself happy and hoping in some small way that I was supporting my favorite idols. But after a decade or so, that little idea turns into a clutter of stuff that at the end of the day, really doesn't hold as much value as it did, say, 10-20 years ago. But, at the same time, it holds enough value that the idea of just tossing it in the trash hurts. Mainly because it's not exactly something that should be thrown in the trash. You can't exactly donate it. If I were in Japan, I'm sure I could find a place, but in the US, not so much. The Book Off, I believe in NYC stopped excepting Japanese CDs a few years ago, or so I heard. Not sure if that's true. So you're left with very few options. And that's where I currently find myself. Sell it? Find some place to donate it? or Trash/Recycle what I can and try not to feel bad about it while doing it.
But on a sunnier note. Media holds up well. I do like the books. Some of the older ones were nice, but it's all about photobooks now and that's not my jam. I'd say the acrylic stands, since they are small, but that's a not something I collected. I do enjoy my keychains and anything USEFUL. I got quite a few coffee mugs. I'd never get rid of those.
And a few dozen t-shirts. Useful everyday items are much nicer to have than stuff that's just going to sit on a shelf and collect dust.