When you use the superlative form of great, you are assuming a great deal, that there is none equal, you are better than anyone else, you have arrived, you have it made and believe it to be true.  Others may doubt your claim, but you are comfortable in your assertion.  And you know what, everyone can legitimately make that ‘greatest’ claim.  How?  Read on.
Because everyone is different.  There is no one who is the same as any other person living or dead.   Every person is unique so it is logical that since we all are made from a different pattern, then we can all claim to be the greatest since there is no one who can compete with our claim. When you at times feel that you are a no body, one left behind, consider this.
Take for instance a skill that someone has mastered and for which that person can rightly claim as being the greatest for instance remember Muhammad Ali (Cassius Clay) the champion boxer who was not ashamed to proclaim to the world that he was the greatest and maybe he could back it up with his achievement in the ring.  So we can understand how he might have bragged about being the greatest.  But YOU also can truthfully claim the greatest status too.  For example, you are a second string football player who can claim no heroics on the gridiron but there is one you can make.  Since you are the only one like you, you can say, I am the greatest football player on the field who PLAYS LIKE I PLAY.  Since no one can duplicate your skills, ergo you are alone in your status and therefore the greatest since there is no one who can claim to be who you are.   This idea can help anyone to gain confidence in being the ‘greatest’ in any endeavor if you simply add those last few words to your claim: no one is as great AS I AM since I am the only one EVER who is or will be like me,   The scope of greatest is unlimited.  Here is another example.
Moderately talented as a piano player, he often felt his accomplishments were far inferior to others with greater achievements.  He often became discouraged when his performance was clearly unremarkable and was subject to bouts of being embarrassed when the mistakes became acutely obvious.  But he had a loyal supporter when was always there to offer encouragement, “Everyone makes mistakes.”  That was enough for him to continue. Then one day, he read a statement that forever changed his attitude about his musical accomplishments.
It was a simple sentence uttered by a character in a story he was reading.   She suffered from a lack of confidence in her piano playing until she came across this one sentence that changed everything.  She concluded that instead of lamenting about her musical inadequacy, she had the right to make the claim that NO ONE COULD PLAY THE PIANO AS WELL AS SHE HERSELF PLAYED THE PIANO. The statement was life changing since now she could lay claim to being THE GREATEST piano player when she added the reservation: who plays like I play.  No one could ever take that title from her; it was an epitomical moment in her life.  
So no matter what your calling, whether a carpenter, lawyer, farmer, mechanic, caregiver, writer or whatever, consider this:  no one can EVER take away your title of your being the GREATEST in your field if you simply add:  no one can overachieve my effort by doing it better than I can do it like I do it.
Bill Lee, PO Box 128,
Hamer, SC 29547