Some items not yet mentioned to throw in for consideration:
Of perma-available items, how about the alien cloths? Alien weapon white bags seem a lot more common than the cloths.
Eg. Malogian Cloths, from the final boss in red:
There’s cloths from the UFO too that, due to 1x UFO per many wormholes, could be equal rarity, or even rarer than the planetary ones.
Of limited availability items:
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Quintessential Quiver (ST that only drops from The Glitch special event enemy):
(I discounted Useless Katana which also drops from The Glitch because many players got given one ‘free’ in the past, see the wiki on it for more info.)
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The Paranormal Wizard reskin STs were I believe only in for one event, last Halloween:
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There’s probably some other cloths that have never been repeated, eg. the wiki has a limited edition cloths section.
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Or yeah probably some obscure char skin hatcher or pet skin hatcher item will only have few people who kept it unhatched, or unfed/undropped (if duplicated). Like I’ve got this wormy dude out of a chest event, there’ll be some bizarre examples like this I’m sure, where people have saved instead of used it, from an event or a shop box: