There's about a million reasons why measuring defense is difficult, but among the most important is this simple question: Which plays are the most important to measure? That's not an issue on offense, where every plate appearance offers the hitter a chance to do something notable. But on defense, it's important to separate the lazy pop fly that's an out 99 percent of the time from the low liner that's caught only 20 percent of the time.
So that's exactly what we've done. Using Statcast™ exit velocity and launch angle -- it's important to use both, because even a 120 mph rocket that is hit straight up will still be an out -- we've identified a range of line drives and hard-hit flies that are a hitter's best friend, the kind of ball that has a .595 batting average across MLB. (We've excluded home runs, for obvious reasons.) Then, we ranked the defenses that did the best at saving their pitchers by preventing those hits.
You probably had some expectations in mind for who'd be atop that list. You probably didn't expect it to to be the Oakland Athletics. Neither did we. Let's get right to the rankings, and then we'll explain how we got there:
Lowest batting average against, 100 mph-plus line drives (10-25 degrees)
1. Athletics, .516
2. Phillies, .530
3. Mariners, .540
4. Reds, .556
5. Angels/Rays/Yankees, .559
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MLB average: .595
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26. Rangers, .650
27. Pirates, .652
28. Brewers/Dodgers, .657
30. Rockies, .662
We're showing batting average here, but another way of expressing this might be "the A's turned 48 percent of these balls into outs, while the Rockies made only 33 percent."

The methodology here was relatively simple. First, we took all batted balls that were struck between 10-25 degrees of launch angle. From those, we took only those hit harder than 100 mph, which was between 200-300 batted balls allowed for just about every team. (The Braves allowed the most such balls, 97 more than the Dodgers, who had the fewest. Unsurprisingly, the 2015 Braves had an ERA of 4.41, nearly a run higher than the Dodgers' 3.46.)
So that was our sample -- nearly 8,000 balls worth -- and our .595 average is another way of saying those became hits nearly 60 percent of the time, overall. Now, let's be clear, here: There's obviously some luck involved. If a particular pitching staff has found an unfortunate skill for allowing line drives that land on the foul line, well, no defense can do much about that. But as we've always known, defense is a team effort, and stealing hits in this manner -- Oakland turned 137 of these balls into outs, while the Dodgers, for example, converted just 70 -- matters.
Now, it may seem surprising to see the Athletics atop this list. After all, Oakland lost 94 games last year and committed the most errors in baseball with 126. But we know that errors are a terribly flawed stat, and even if they weren't, a whopping 59 of them came from just two players: infielders Marcus Semien and Brett Lawrie. Since the overwhelming majority of the balls in our sample ended up in the outfield, the A's infield issues don't really apply here.
When looking at just the outfield, it's a different story. The A's outfield finished just 13th in Ultimate Zone Rating and 12th in Defensive Runs Saved, but that's partially because they were poor at two of the three primary inputs: arm value, where they were in the bottom five and had no one in the top 55 of our recent arm strength preview; and error runs, where Inside Edge ranked them dead last in plays marked "routine," or 90-100 percent likely to be made. 
But when it came to the spectacular play, well, that's where the A's made up for it, thanks in part to plays like this, perhaps unsurprising given Billy Burns' elite running speed…
Gif: Billy Burns Amazing Catch OAK
...and this, from Sam Fuld:
Gif: GIF OAK Fuld Diving Catch 91815
...and this, from Josh Reddick:
Gif: Josh Reddick Diving Play OAK
By Inside Edge's metrics, the A's were in the top four in percentage of plays made marked Remote (1-10 percent), Unlikely (10-40 percent) and Even (40-60 percent), which aligns with what we have here. They made far too many mistakes in pretty much every other aspect of the game, leading to that last-place finish, but in terms of stellar outfield plays, they were unmatched.
We don't know yet how this will correlate year-to-year, though new acquisitions Khris Davis and Chris Coghlan aren't known for their range, so the smart money is probably against it.
Still, it was an exciting bright spot in a poor season. And as for the 30th-place Rockies? Even when excluding home runs, they're still finding trouble turning balls into outs, though in this case, it's not due to the thin air so much as it is to the response to it. Coors Field has baseball's biggest outfield, in an attempt to make it more difficult to hit the ball out of the park. The unintended consequence is that there's more room for non-homers to land. There's just nothing easy about trying to prevent runs at altitude.