Angels first baseman Albert Pujols might be the best right-handed hitter ever. It's hard to prove, but very possible. Unfortunately, there's just nothing productive to be done with such a debate. In that contest, Pujols would square off (primarily) with Hank Aaron, Rogers Hornsby and Jimmie Foxx.
Those players could not all be more dissimilar, but not only in their approaches or the composition of their numbers. If it were that easy, one could safely choose the one over the others. However, each man played in such different eras that no direct comparison is possible.
Pujols' prime came in an offense-friendly era, one of the best ever. Aaron played through three distinct phases of the historical run-scoring cycle. Hornsby dominated the National League in the 1920s unlike any player ha sever dominated any league, but by general acclaim, they were weak opponents upon whom he trod.
Aaron had to deal directly with scrutiny based on the color of his skin, playing (as he did) shortly after baseball became an integrated game. Hornsby never had to face an African-American pitcher, but also never got to play regular night games.
Comparisons across eras in baseball call into question variables utterly impossible to quantify. Lining up the best hitters of all-time is an exercise in vapid punditry, and no one needs it.
It is possible, though, to figure out who were the best batters on individual eras, to find the proper order in which to place (say) Barry Bonds, Frank Thomas, Jeff Bagwell, Edgar Martinez and John Olerud, because those men performed under the same basic conditions and against essentially the same competition as one another.
That's the exercise in which we now partake. Here are the three best hitters of each decade of MLB history, since 1900.