Boxes. I like pretty boxes. I have boxes for pieces of gear I don't own anymore. I'm going to need to find a better way to store and display them, as the post-modern sculpture I'm slowly building out of them is taking up more real-estate than the gear itself.
I also have a growing collection of dropper bottles. I don't know why. I do the same thing with unique alcoholic beverage bottles. I think I just like the aesthetic of the bottles and my instinct is to not get rid of a nice glass container.
Close second would be atties. It just makes the most sense to me. It's harder for me to justify buying new regulated devices and mechs when the small collection of ones I have already work great. Three regulated devices and two super-solid mechs seems sufficient. Atties are less expensive and all of them have their own distinct performance profiles. I'll buy them just to try them. Half of them don't stick around, but for me, it's just about trying something different.
I suppose you could say I collect batteries, too. I have 30 or so at this point. I'm considering categorizing them and keeping a clipboard with checklists nailed to my vape station just to make sure they all see decent use/stay charged. I need a battery schedule, heh. In all fairness, I think batteries are good to have in excess. Never know when you're gonna need to pull one out of rotation. Every time I get paid, I've gotten into the habit of buying two. They're cheap enough and its always nice to have brand new batteries.
I think part of the reason I have so many is that l end up lending a lot of them out. I've lent a few out and gotten two different batteries back because the one I lent-out got damaged. My friends seem to value them more than I do, probably because they don't have enough backups...
Generally, I try not to collect gear, though I've gone through my share of it. If I have a piece of equipment that has served me well in the past and I upgrade, then it's next in line to be a hand-me-down to either a vaper in need or a smoker who's interested, but not quite convinced that its worth the investment. No sense in having perfectly good equipment if it's just sitting there.