Another Holy Grail of mine is probably a 2013-W gold 1/4 eagle.  I received this from a friend who didn't know the value.  I proceeded to tell her, and she said that as a present for telling her, I could have the coin.  :)

my favorite is a set. the march of dimes limited edition dollar with the 2 dimes. I love tyhe hand holding the baby on the back. My holy grail would be a CC morgan.

Holy grail would be discovering a new Lincoln Bicentennial DDR. Also the 1849 double eagle and the 1964 peace dollar

@cuts
@luv2collect
My only true "grail" is a 1955 poor man's DD. It's in really good condition. I think it's worth $200... and I found 2 in the same coin roll! ;)
 I just looked that one up and learned something. Very interesting! 
Unfortunately, this is just die abrasion, so its not very valuable. I also cherry picked one. Dealers made it popular.

My favorite coin is my Lincoln Cent that has bubbles from the mouth all the way to the date, I call it The Spitting President or SPOTUS.  My Holy Grail? Either the 1893S Mint State Morgan offered by Legend Numismatics years ago - or - The highest graded Gold Slug ever that I was privileged to view at Kagins booth with Mr. David McCarthy. 

Great question. For myself an extremely tough one since I have two which are equally holy-grail coins. As one who leans towards the dime, it would be an 1873cc No Arrows followed very closely by the obvious 1894s. Somehow I don't think hitting the PowerBall would be enough to buy the former.

My current Grails (It's a constantly shifting target) are the Newfoundland $2 Gold, the New Brunswick Half Cent, Half Dime, and Dime.  If I get all these, I will have a complete type collection of Canadian Provincial Coins. 

I recently joined the ANA after a 42 year break! ;)  I saw this thread and thought it would be a great way to join back in the fun.I have two favorite pieces both winged liberty mercury dimes.  One is a 1916-d graded G6 and the other is a MS 1945 showing faint split bands.
My holy grail would be a high-grade circulated 1916-d mercury dime, about XF40 - AU 53 or so.
I am also quite fond of my 1928 Peace dollar which is a grade 1 or a 2. :)

Mercury dimes and Peace Dollars. You are my kind of guy. I would add Bison nickles.

@Longstrider
Mercury dimes and Peace Dollars. You are my kind of guy. I would add Bison nickles. Agree about the nickels!  I just got three rolls to look through, haven’t done that since 1977!😄Here’s my low grade Peace dollar, forgot to add it before.