In early March, Statcast™ introduced Catch Probability, which allows us to evaluate outfield defense in an entirely new way. It can measure an outfielder's starting position and the opportunity time of the ball from pitch release, allowing us to look at how far he had to go and how much time he had to do it in. From that, we can say how likely it is (or isn't) that other outfielders make the play given the same opportunity. Soon enough, that will include running direction and proximity to the wall, too.
Right away, we've had some fun with this, showing how unbelievableBilly Hamilton is, why Kevin Kiermaier was worth his big contract extension, why the Twins' outfield should be a lot better in 2017 than it was in '16, and which outfields did the best overall at making highlight plays. You can check out leaderboards for yourself right here. We'll be doing a lot with this in the season to come.
So far, so good. Now let's take it a step further. If we know the Catch Probability of a batted ball, then it's simple enough to add a value to it. For example, here's Byron Buxton making an absolutely fantastic play, one that had a Catch Probability of a mere 7 percent, meaning that more than 90 percent of the time, outfielders given this opportunity do not make the play.

Surely, Buxton deserves more credit for making this extremely difficult play than for catching an easy pop fly that's caught by virtually every fielder every single time, right? Of course he does. So given the fact that this ball drops 93 percent of the time, and he made the play, he receives a +0.93 for it. If it were an easy play that drops only 3 percent of the time, he'd get merely a +0.03 for it. It's a simple way of assigning credit not just for the number of catches made, but the difficulty. 
Given that, it's easy enough to accumulate all the balls over the course of a season and give a player a simple plus/minus score, which accounts for plays made above average, relative to that position, rather than all outfielders. So let's limit ourselves to only outfielders who had at least 100 chances and check out each of the three positions. Who, in 2016, did the best at hauling in the balls they needed to?
Left field
+12 Adam Duvall, Reds
+9 Christian Yelich, Marlins
+8 Brett Gardner, Yankees
+6 Angel Pagan, Giants
+5 Michael Conforto, Mets

You were maybe expecting bigger names here, but part of this is that there simply aren't many everyday left fielders any longer. As the position has become a home for platoons, only 12 players took 400 plate appearances as a left fielder last year, compared to 18 in center and 21 in right.
That said, don't let the relative lack of competition undersell what was a surprisingly good rookie season for Duvall, who started the year in something of a job competition with Scott Schebler and ended up hitting 33 homers and making the National League All-Star team. Of our 30 left fielders, Duvall made the catch on 84 percent of the balls that were hit his way, second behind Alex Gordon's 85 percent and well above the 71 percent that the lowest-ranked left fielder (Robbie Grossman) captured.
Remember, though, we're giving credit for difficulty here as well, and while Gordon (who missed time due to injury) managed only six catches with a Catch Probability of 50 percent or lower, Duvall managed 15 of them. Like, for example, this diving stab against Milwaukee's Hernan Perez:

Now, if you were surprised to see Pagan over Starling Marte, know that we were, too. That said, a big part of Marte's value came from his elite arm, which was measured on max effort throws with a 97.1 mph average, the best of any regular outfielder in baseball. We don't have arm value rolled into this just yet, though that's on the road map.
While the still-unsigned former Giant, however, tracked down a slightly lower percentage of balls than Marte, he did make more outstanding plays, like this:

Center field
+20 Hamilton, Reds
+18 Ender Inciarte, Braves
+11 Kiermaier, Rays
+10 Kevin Pillar, Blue Jays
+9 Jake Marisnick, Astros

Now we're talking, because these are exactly the names you'd expect to see on any list of baseball's best defenders. We dug into Hamilton extensively already, and you know Inciarte, Kiermaier and Pillar, so Marisnick is the interesting name here. Despite a mere .268 career OBP, he's valuable to Houston because his glove is so elite. Fifth here among center fielders even though he isn't an everyday player, Marisnick is sixth among all outfielders in plays made.
"How come Kiermaier isn't higher?" you may be asking. His overall catch rate of 94 percent was the highest of any of our outfielders, it's true. But -- and this is another topic entirely, that we'll dive into separately -- Kiermaier also had, on average, the least difficult batted balls in baseball coming his way. He still outperformed that, because he's great, but he also had barely one-third of the Five-Star opportunities that, say, Michael Trout did.
Besides, Kiermaier is also a great example of "amazing plays that don't require a dive," like this robbery of Jacoby Ellsbury. It's not as spectacular looking as Hamilton or Pillar, but it didn't need to be, and that's the point.

Right field
+22 Mookie Betts, Red Sox
+21 Adam Eaton, White Sox (now with Nationals)
+17 Jason Heyward, Cubs
+10 Peter Bourjos, Phillies (now with Rays)
+6 Steven Souza Jr., Rays

Betts isn't just the top right fielder here, he's also the third-best outfielder in our metric overall, a good reminder that he a superstar in every facet of the game. (Remember that these numbers are specifically relative to each position, so while Betts' +22 is the highest number you see here, the bar is lower in right than it is in center, so when compared to all outfielders, Inciarte (+25) and Hamilton (+24) just barely edge him out.) 
Allow this to also remind you that despite his hitting troubles, yes, Heyward remains a truly elite outfielder, but also to point out that the Rays may be putting together an insanely good defensive outfield. In addition to Kiermaier and Souza, they added Bourjos from the White Sox last week, Mallex Smith from Atlanta and Colby Rasmus from Houston. Assuming Corey Dickerson spends most of his time at DH, the Rays have what might just be baseball's best defensive outfield.