Bravo. It was my Sicilian grandfather who inspired me to collect coins. He was bright and savvy enough to collect coins that would astound PCGS graders even now. He taught me to never judge a book by it's cover.
Your grandfather is a very lucky guy, bro. There's the most important reason of all that I expressed that in the present tense, and not the past, even tho it's obvious he's passed on. That Silver dollar has his psychic imprint on it. He'll live forever because you remember him. For your granddad, a quote from an Asian Indian fellow: " Death is only the turning out of a night light because there's no longer any need for it, now that an eternity of Sunrise has dawned. " Our lives have meaning being brief candles.
My greatest and most favorite (non Mint Error) is this $5 Gold that I won in a Contest here on Cointalk a few years ago. I even sent it to NGC to grade it -
When I was very young, while walking home from school, I found a penny. When I got home, I showed my dad. The next day he took it to work with him, and this is what he brought home. We worked on railroad at a local steel mill. I have kept this for over 50 years.
Mine would possibly be my 1955 Bugs Bunny variety graded 66FBL by NGC. I think I’m biased towards that particular coin because I picked it raw at a show.
How do you define "greatest"? Oldest? Lowest population certified? Lowest mintage? Highest grade? Most Valuable? My favorite? The one I'd like to show off? The one with the best toning? The most impressive? Best variety? Best variety that I discovered? The one with the best story? The most historically significant? Pick one, and I'll show you mine.
Though I'm not completely sure what is meant here by "greatest coin," one of my favorite coins has to be a 1794 Large Cent, even despite its less than impressive condition. It dates to George Washington's presidency and to when the country was just getting started as a small republic. It's awesome to contemplate what this thing has seen over the last few centuries, along with all of the people it has outlasted. Did anyone spend it? For what? What happened to it after that? Etc. Impossible to know, of course, but intriguing to speculate on.
My favorite coin is this 1964 D Washington Quarter, struck through cloth. I believe the reverse has a misaligned die as well. I bought it raw and sent it in for grading and error attribution.