This is a question that is difficult to answer objectively...and even the posted explanation creates conflict. The one knife that HAS to be in my collection is not necessarily one that I would say every knife collector should own. I've got knives that I value much more than their monetary worth - but I don't know that I consider them must haves for myself, let alone anybody else.
As much as I like my various folders - modern and traditional - my obsession is fixed blades...so I would likely dismiss folders from this game (even the ones I ones that have "permanent" status). That's sort of a cop out, I know; but I've tried to identify a folder that I feel that way about, and I don't think I can. Going back to fixed blades, though - I'm a big fan of the RMD - I've got a small pile of them, with a couple more already ordered. I have 2 Randall knives - received about 15 yrs ago. My desk has stands which hold 2 custom handled MBBs - an M4, and an M10 (cleaver style). I've got one offs, and favorite users...but the must have, for me, would probably be the Gerber Mark 1. I had a standard and a presentation grade for many years; and I thought I left them behind, or otherwise lost them, in a move. I ended up obsessing on them until I'd replaced them both. (I later recovered the first 2; so now I have 4...5, if the 35th Anniversary edition counts.)