This is a tough question. People say Ruth a lot, but I disagree. I like the above picks of Rose and Aaron.
To me, the best baseball player of all time should be able to do it all, or at least most of the game. Rose for example could do it all except hit for power.
If youre going down as the goat baseball player, you need to be a 5 tool player or as close to a 5 tool as possible. A great hitter who is an average fielder is not the greatest baseball player who ever lived I dont think.
Had a friend who said Willie Mays, thats long before my time, but his reasoning is that Mays could do it all. But its funny how much more difficult this question is than the basketball or football question.