The game of baseball has a deep tradition of accumulating statistical data. Here are the 5 most important statistics in baseball as a measure of effective performance at both the player and team levels:

Wins Above Replacement (WAR): WAR is a metric that attempts to measure the total value of a player to their team. It takes into account a variety of factors, including batting average, on-base percentage, slugging percentage, home runs, RBIs, stolen bases, fielding percentage, and baserunning ability.
On-Base Percentage (OBP): OBP measures how often a player reaches base safely. It is calculated by adding a player's batting average and their on-base percentage, which is calculated by dividing their number of walks and hit-by-pitches by their plate appearances.
Slugging Percentage (SLG): SLG measures how often a player hits for power. It is calculated by adding a player's home run total and their total bases (the number of bases a player reaches on a single, double, triple, or home run) divided by their at-bats.
Earned Run Average (ERA): ERA measures how many earned runs a pitcher allows per nine innings pitched. Earned runs are runs that are scored without the benefit of an error or a fielder's choice.
Walks plus Hits per Inning Pitched (WHIP): WHIP measures how many walks and hits a pitcher allows per inning pitched. It is a good measure of a pitcher's control and ability to prevent baserunners.

These are just a few of the many statistics that can be used to measure a player's or team's effectiveness in baseball. The specific statistics that are most important will vary depending on the position of the player and the role they play on the team. However, the statistics listed above are a good starting point for evaluating the performance of players and teams in baseball.