Back in 1987, when I was 15 years old, I wrote to Muhammad Ali asking for an autograph. This is what he sent back. At the time, I thought it was really unique, and really special. Over the years I have seen many things he signed like this, so while it might not be unique, it's still very special.
But what really amazed me at the time, and still does, was that the letter I sent him meant to say "...please send me an autograph for my collection..." but I mistakenly wrote "...please send me an autograph collection". And you know what, he did! In addition to this, he sent an autographed pamphlet on Islam, and signed the letter I sent to him. And addressed the envelope to me himself.
He was The Greatest in many ways: as a boxer, humanitarian, inspiration. But even to a kid writing him for an autograph.