If you were asked who baseball's best defensive outfielders were, you might immediately think of Kevin Kiermaier or Mookie Betts. Maybe you'd go to the breathtaking speed of Billy Hamilton or Byron Buxton, or the regular highlight-reel appearances by Jackie Bradley Jr. or Adam Engel, or how easy Ender Inciarte makes everything look, or the consistent yearly success of Jason Heyward or Lorenzo Cain.
We're guessing that if you weren't a Cardinals fan, you probably wouldn't have included Harrison Bader in that list. It's time to fix that mistake. If it's maybe too soon to call him baseball's best defensive outfielder, we can at least describe him as the best outfielder you don't know enough about. He's been that good.
This is going to be one of those cases where the metrics and the eye test align very well, so while we'll throw all sorts of numbers at you in a second, let's start with some eye candy. Here's a diving Bader catch from last month, when he robbed a hit (and almost certainly saved a go-ahead run) from Buster Posey. It looked great. Most Bader plays look great.

But merely having made a diving play doesn't make a catch great, because lots of diving plays can start off with unimpressive beginnings -- a bad route, a poor reaction, slow foot speed, and so on -- and this is where the metrics come in.
Based on the fact that Bader had 4.7 seconds to run the 92 feet required to get to the ball, and it wasn't a play impacted by direction or the wall, that catch had a 13 percent Catch Probability. Another way of saying that is that nearly nine times out of 10, other outfielders didn't convert similar chances. Compare that to this diving catch by Rhys Hoskins -- a converted first baseman with poor footspeed -- that looked nice but is caught 97 percent of the time. All diving plays aren't created equally. The eye test doesn't always work here.
That allows us to credit outfielders differently toward a season-long score, too. Despite the dive, Hoskins doesn't get a lot of credit for making a catch most fielders make (+.03, or the remainder of 97 percent). But Bader gets a ton of credit for making a nearly impossible play (+.87, or the remainder of 13 percent). Do that for every play over the course of a season, and that's how we get to Outs Above Average, our aggregate range-based metric.
Now, because outfielders don't all get the same amount of playing time, there's two ways to show how valuable they've been. One way is with a counting stat like Outs Above Average (think home runs or stolen bases), which tends to favor everyday players. Another is with a rate stat (think batting average or ERA) that allows for part-time players to rank highly.
Bader has started only 56 of the 119 St. Louis games so far this season. He's at the top of the leaderboards in both. In fact, he's at or near the top of so many leaderboards, we're just going to throw them at you in a row. There's a lot to like here.
Bader is elite in Outs Above Average
As we said above, OAA is the Statcast™ range-based metric for an outfielder's seasonal contribution. Bader is tied with Hamilton atop the leaderboard, despite having played nearly 400 fewer innings.
Outs Above Average leaders, 2018
+16 Harrison Bader
+16 Billy Hamilton
+15 Adam Engel
+14 Ender Inciarte
+13 Lorenzo Cain
+10 Jackie Bradley Jr.
+10 Albert Almora Jr.
+10 Michael A. Taylor

Bader is elite in Catch Percentage Added
For players who aren't in the lineup every day, it's sometimes better to look at it in a way that's not a counting stat. In this case, it's relatively simple: You look at the difficulty of the balls hit to a fielder and say how many of those balls an average outfielder would have caught, compare those to how many balls the fielder in question did catch, and see how much value was added (or not).
In Bader's case, an average outfielder would have been expected to catch 84 percent of the balls hit to him However, he's actually caught 94 percent of the balls hit his way, meaning he's added +10 points of value. Among the 151 fielders with 50 chances, that's the best. (At the other end, Daniel Palka was expected to catch 86 percent of his chances, but he's collected only 71 percent, a worst-in-baseball minus-15 points.)
Most Catch Percentage Added, 2018
 +10 Harrison Bader (84 percent expected, 94 percent actual)
+7 Jake Marisnick (89 expected, 96 actual)
+5 seven players tied, including Inciarte, Cain, Hamilton, Engel
Bader is elite in Sprint Speed
So: How is Bader doing this? Obviously he's fast, because you can see he's fast with your own eyes. But you might not know just how fast he is. We measure that with a metric called "Sprint Speed," which is intended to find a player's top speed in feet per second. Bader is tied for third in baseball, right up there with the most notable speed demons in the game.
Highest Sprint Speed, 2018
30.5 ft/sec Byron Buxton
30.1 ft/sec Billy Hamilton
30.0 ft/sec Harrison Bader / Adam Engel / Delino DeShields / Trea Turner / Magneuris Sierra
29.8 ft/sec Ronald Acuna, Jr.
(Major League average: 27 ft/sec) 
When MLB.com's Joe Trezza asked Bader about elite speed in June, he received the perfect answer.
"What you need to be asking is, 'When did everybody else learn that I was fast?'" said Bader. "I've been fast for a really long time. These new metrics are helping everyone see that."
That's a lot of numbers, so let's go back to the video to show you just how impressive he's been. Last week, Bader came flying in from center to rob Martin Prado in Miami; as has become usual for him, it looked great and the data backed it up, calling it a mere 15 percent catch, because he needed to go 80 feet in 4.3 seconds.
Let's compare it side by side with a very similar opportunity that Kiermaier faced last September, when he needed to go 81 feet in 4.3 seconds, in the same direction, off the bat of Gary Sanchez. Kiermaier is undeniably one of baseball's great defenders, but he didn't get there. He didn't even come close. That's not to say that Bader is better than Kiermaier, but that he's making the kind of plays that even baseball's best defenders can't always get to.

Bader also has the fastest individual Sprint Speed tracked by a Cardinal since Statcast™ came online in 2015, and 15 of the team's 20 strongest outfield throws this year, topping out at 98 mph to prevent Raul Mondesi from trying to score. Remember: three different Cardinals outfielders,Marcell Ozuna, the since-traded Tommy Pham and William Fowler, have more playing time than Bader does this year.

Despite spending Spring Training making ridiculous catches in Florida, Bader didn't even make the Cardinals' Opening Day roster, losing out to Yairo Munoz. It's really only been since Pham was traded to Tampa Bay on July 31 that he's been in the lineup regularly, taking over center field, and while we've been focusing on his defense, his .275/.343/.425 hitting line makes him 10 percent better than league average. (Not to mention that he's apparently had something to do with Paul DeJong's recent hot streak.)
No one really saw Bader coming, as he never ranked terribly highly on any national prospect lists. Even on the Cardinals, he was somewhat lost in a flow of outfielders, not just Ozuna, Pham and Fowler, but also Jose Martinez and Tyler O'Neill. They wouldn't have thought that they had, essentially, "Billy Hamilton-but-if-he-could-actually-hit" on the bench. So far, it seems like they do. It's hard to see him going back there any time soon.