I think this question is a matter of personal interpretation, however... For a pitcher, typically the ERA, or Earned Run Average, is the given the first look. The ERA corresponds to the number of earned runs given up based upon a complete nine inning game. Every run that scores though is not necessarily considered "earned". For example, if a batter reaches base on a fielding error, that base runner, if he scores, is not counted against the pitcher as an earned run. For a batter, typically the BA, or batting average, is given the most weight. Batting average is simply calculated by taking the number of hits divided by the number of at bats. Again, like the ERA, not every "appearance" is considered an at bat. As an example, if a batter walks, it is not counted as an at bat for that hitter.
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The most important stat in Baseball is on base percentage also known as .OBP. It shows how often a batter reaches base. Joe Mauer had the highest on base percentage in 09', at .444. That means he reached base almost 4.5 times for every 10 at bats, The highest on base percentage in history for one season was set by Ted William. It was .553, according to World Baseball Almanac's website. The reason this statistic is so important is because it shows how often you have helped your team, and given them a chance to bat you in.