A most wanted list. Where do you start? No one item, more like multiple items to fill the gaps. Bearing in mind, if we found every item we looked for, then we'd have nothing to do and would soon become bored just looking at it all. It's the the thrill of the hunt that keeps you going, whether it be the hunt for items or just finally cracking that research question.
The reason gaps exist in the collection is because they are very hard to find, so I guess they are all holy grails to varying degrees. Mine are all Soviet/Russian.
Suits: The first G suit, PPK-1, and the first partial pressure suit, VKK-1.
Helmets: ShLO-50 early mesh top leather helmet. ZSh-5S and ZSh-5B armoured combat helmet. ZSh-7AN, ZSh-7APN, ZSh-7B, ZSh-7AS,
O2 masks: KM-14, KM-18, KM-18N, KM-24, KM-24N, KM-30 (not 30M).
I haven't included items that are most likely unobtainable. I know where two of the helmets on the list are. It's just a matter of trying to justify the huge cost of buying them.