New Nationals manager Davey Johnson is one of the more accomplished managers in the history of the game, having taken three of the four teams he's managed to the playoffs, winning AL Manager of the Year honors in 1997 and leading the Mets to a World Series title in 1986. And by at least one measure, he's done it by taking teams to more victories than they should have had.
In six of his 13 seasons as a manager, Johnson's teams have finished above their Pythagorean Win Expectation, which measures how many games a team should have won by looking at its run differential. It's not necessarily indicative of what a manager got out of his team - luck and chance are powerful agents in a Pythagorean record, too - but it's interesting to note that Johnson's teams are nine games over his Pythagorean expectation for his career. in his first season with the Mets (1984), the team was outscored by 24 runs and predicted to finish 78-84. It won 90 games.
Jim Riggleman, by comparison, is a collective 32 games under his Pythagorean expectation for his career, having managed just three of his 12 teams to win totals that exceeded their Pythagorean record (though we won't get on his case too much for finishing 4-8 with the 1992 Padres, a game under their expected win total for those games).
Again, Pythagorean expectation isn't a perfect predictor of a manger's ability to get the most out of his players, and there are plenty who would argue the best thing a manager can do is stay out of the way of his team's ability to win. But at the very least, Johnson has proven an ability to do that.