He won rookie of the year, he hit 18 home runs his rookie year. He also is in the top ten in batting averages. He is in the running for most home runs. He has also been excellent in the field. He was even nice enough to ask for a fans phone before a game, and took a selfie.
If I need to actually answer this with facts then I am not sure if you have ever watched baseball before. He is a very good player, having a tremendous season, but to even say he is in the top 1,000 players of all time is pushing it. Baseball has been around for over 100 years and Harper is no where close to being at the top
If I've never heard of him, how in any way could he be the best ever? There are many baseball players almost everyone has heard of, including Babe Ruth, Lou Gehrig, Jackie Robinson, and Derek Jeter who would be much better examples for the best baseball player ever. Perhaps sometime in the future, this guy will become the best ever, but pros before him have set such a high record he needs to beat.