The rarest music library I've ever heard just so happens to be MUTEL, in which its hundreds of cues can be heard in various TV productions during 1950s and 60s.
Since I’ve focused most of my collection on Capitol it would definitely be Hi - “Q” reels L-205/206 by Jack Mayborn and X-261/262 by Bent Myggen. Took me YEARS to find them.
The 27th Dimension music library.
One day I was searching for Space Visitors (Illustra-Son IS 89 05 01) on browser and then I found the CD on sale on Rakuten
The rarest I found was some Gerhard Trede CDs. It is difficult to find rare stuff.