Very curious that only two replies mentioned a non-US coin.  I would have expected more.

I cannot say I really have a Holy Grail coin which I desire.  I am happy to get what I can find.  I do have a couple of items which I have picked up along the way and which I am happy to own:

A 1702 Queen Anne threepence of UK in XF condition and a 1899 Morgan Dollar in G condition.  Why am I so glad to have the latter?  Because in G4 is listed in the catalog at $130 and I found it in a roll of culled dollars I bought at spot price.    

I also have a Morgan dated 1848, obviously altered, but an interesting talking point as it looks perfect.

As my wife put it I am like a dog with a bone. I started a errors set in my collection and stated with a waffle set one of each denomination and now its a few more than that I have started collecting quarters that can be identified by state and found they need to be certified n order to be sure as i got a quarter on ebay that was supposed to be a state I wanted and under a big eyed glass it was blank no design. so 50 states is going to stay my grail for a while! :-)

I actually donated two of my Holy Grail specimens to a worthy museum and established a legacy for my family name.

I like your pieice Its a geat looking specimen - I never got that enthralled with gold I always wanted a more tactile item so larger coins were my primary interest. Everybody has his bent in life! ;-)

For simple and clean lined beauty nothing can compare to ancient Greek coinage. Each coin is unique because each piece is done by hand and hammer. For me the coinage of Terina display superior craftsmanship in the die engraving. Keep in mind there are no machines to aid in the production of the dies. This example displays great metal, clean with fresh dies and slightly over 1\4" relief. Now if I could only win the lottery!

My "holy grail" is a 1792 Getz pattern, in copper, but silver would be even better.  Will probably never obtain either.  

Thank you to everyone who has shared so far! I've enjoyed doing some research on the different pieces being mentioned and trying to edumacate myself. Keep 'em coming!

Thursday night at the Great HoustonCoin Club meeting a new polymer $20 was on display so with a little crowbarring in the wallet I added it to my collection. New Grail? Whatever acquisition is just made becomes the new grail? 

My Holy Grail coin is the 1780 Maria Theresa Thaler  struck n Prague ( in 1780) Only two are known to exist. One is in the Vienna Coin Cabinet in Austria , and rumor has it the second  sold for €25000  sometime ion the last 20 years. It is very different to the restrike having the mint signature Evs - IK. The Restrike has PS - IK

Well, over the past few weeks I've managed to obtain a couple of my Grails. I picked up the 2011 25th Anniversary Silver Eagle Set and the 2015 Gold Liberty High Relief. Beautiful coins that I am very happy with. I'm trying to find some time to post some pics.

I came close to getting the 2008 W Eagle with 2007 reverse, but the seller just didn't come down to where I wanted to be at. I'm going to keep hunting for that one. I'm also eyeing the 2001 Last Silver Franc now.