In Little League, most kids want to play the infield because being in the outfield just isn't cool enough.

There's no turning two or picking off overaggressive baserunners in the outfield. There are, however, plenty of opportunities to drop fly balls, not to mention endless daisies to be plucked.

But when kids get older, the outfield becomes more important. The bigger they get, the farther their opponents hit the ball, so there's plenty of action beyond the infield.

In the majors, it's the big-time athletes who man who the outfield spots. From Ken Griffey to Mike Trout, some of the best and most popular fan favorites have wowed us with their cannon arms, daring dives, and jet-propelled hops.

How do we separate the best from the rest? Look no further than three letters: U. Z. R.

UZR is short for ultimate zone rating, which is FanGraphs' way of putting a run value on defense, quantifying how many runs a player saves or gives up while in the field. According to FanGraphs, 0 is average, 5 or more is above average, 10 or more is great and 15-plus is Gold Glove caliber.

Over the course of a player's career, their UZR is calculated, but there has to be a way to account for the number of seasons, games, and innings played. That's where UZR/150 comes in.

UZR per 150 scales ultimate zone rating back to an average number of chances per season (150 games) to level the playing field among players at the position and across different seasons.

For this reason, UZR/150 is the determining factor in the order of these 20 outstanding defenders. The only provision is that each player must have at least three full big league seasons under their belt, which eliminates newbies like Odubel Herrera and Nomar Mazara.

Got it? Good! Now it's time to roll on with the list!