Baseball: On-base percentage is the percentage of times a player reaches base on either a hit, walk, or by being hit by a pitch.
Football: DVOA (defense-adjusted value over average), which compares a player’s success on each play to the league average based on a number of variables including down, distance, location on field, current score gap, quarter, and strength of opponent.
Soccer: Ball possession as the percent of the time that each team has the ball.