I don't really have just one holy grail gun. Would like to buy several very nice custom user type guns, and have plenty of time, and ammo to really get to where I could use them very well.
Would love another pristine 1903 springfield, Tommy gun as mentioned before, maybe put together a collection of all of the MBRs, and side arms, the US military has used since the civil war.
I have always enjoyed shooting long distances, and have always had the bargain basement budget to try to do it. Might be fun to get a real world class version to play with.
In all reality, I really do just like guns. Having the money to scratch the gun itch, frequently sounds like a good problem to have!