I don't really have any stand-alone "holy grail" tools.
That said, I am an obsessive Xcelite collector, and I have several drivers that I have been looking for for years to fill out my collections:
-Stubby Black-Handle Nut Driver, Model S6 (I've seen this one twice in large lots on ebay, but the lots always sold for more than it was worth to me)
-Extended-Length T8 Torx Screwdriver Model XTD8XL (only ever seen this one in catalogs, never seen the actual driver)
-Tamper-Proof T10 Torx Screwdriver Model XTD-10TP (only ever seen this one in catalogs, never seen the actual driver)
-Any Extended-Length nut driver from A-13 to A-15 (I have A-16), and any above A-16 (I don't know which of these exist) These can also be labeled L-11, etc. A-** and L-** are mostly the same.
-Any screwdriver/nutdriver that has a part number that ends in "CM". All I know about the CM drivers is that their colors are non-standard. I don't know what "CM" stands for.
Xcelite tools are hard to collect. Mainly, because it's hard to know what they even made. There's a handful of catalogs out there, and they made a lot of custom tools too. Most of the time I don't know I want something until I see it. Like in the case of the A/L Series nut drivers, I had a full run up to A-12, and that's all that's ever shown in catalogs. I thought it was complete. Then I found an A-16?! So I don't know if 13-15 exist. I won't know until I find them.