Claiming anyone is the "best ever" is pointless. There's no way to compare players of different eras, and there's no way to know for sure. Stats never tell the whole story, and some of the "best ever" athletes would not be considered as such if they were on bad teams their entire careers.
Example -- Jim Brown is "arguably" the best running back in the history of the NFL. He played on the Browns when they were a marquee team in the NFL and had a hell of a good offensive line (it's criminal that Gene Hickerson is NOT in the Hall of Fame, by the way), but would people still consider him the best running back in the history of the NFL if he played with the current Browns who have stunk up the NFL since being relaunched in 1999? Probably not.
Likewise, Andre Thornton was a very good power hitter in the 70s and 80s. But because he played on very poor Indians teams, his statistics were lower than they might have been had he been on the Yankees during the same period of time. I'm not suggesting that Thornton was anywhere close to being the "best ever," but he might have become a Hall of Famer if he was on a team where opponents could pitch around him constantly like they did.