I don't know. While it's fun to talk about who is the greatest of all time, it's really hard to compare players from different eras--much more difficult than, say, basketball--not because just the way the game is played has changed, but because there have been MAJOR rule changes that have practically rendered it a different sport.
Assuming we call from, say, 1920 on the "modern" era of baseball, it becomes a little easier, but certainly there are changes that make even comparisons from 1920 on very difficult. Moreover, how do you determine what makes one player better than the other when they do different things? Or how much weight do you give to fielding? Certainly Rose was one of the best hitters ever, but he also played nearly every position on the field--how much weight is that given? Tough to say. But fun to talk about.
True, but let's confine the discussion to just a Ruth vs. Bonds comparison. They were both outfielders and power hitters, so this isn't exactly an apples and oranges comparison. Let's take a look at their numbers, including Bonds' steroid-helped years.
Batting ave: Ruth .342, Bonds .300
On-base %: Ruth .474, Bonds .442
Slugging %: Ruth .690, Bonds .611
RBIs per 164 games: Ruth 143, Bonds 110
HRs per 164 games: Ruth 46, Bonds 42
Fielding: Ruth has the edge in arm strength, Bonds has the edge in speed. Both could play the field.
Pitching: Ruth was one of the best pitchers of his day, and would've likely made the Hall of Fame had he stayed exclusively at that position.
My vote goes to the Babe.