I think comparing across eras always produces apples-to-oranges results, but in terms of impact on the game while playing, Cobb is King, hands down.
He was to his era what Bonds is to this one -- simply the most dominant player. They were/are also among the most polarizing personalities.
Can't say the same of Kaline, Gehringer, Greenberg et al, who were also superb players in their own rights but never considered the greatest performer in baseball during their playing days.
Speaking of great Tigers -- I love Bill James' analysis of Cecil Fielder, it's one of the shortest in his book: Fat guy who hit a lot of home runs for a few years. Spot on.