Gary Sanchez has the best throwing arm of any catcher in baseball, though by only a hair over Martin Maldonado. No one gets the ball out of his mitt more quickly than Salvador Perez, which you may have expected, or Chris Stewart, which you may have not. Yet when you add it all up, it's not any of them who get the ball to second base the fastest. It's Austin Hedges, ever so slightly over J.T. Realmuto.
We know these things because Statcast™ measures them. Now you can know them, too. Over on Baseball Savant, our new catcher defense leaderboards are live. You can find data on arm strength, exchange time and pop time, for each of the past three seasons. Go have fun.
Let's take a second to explain how each of those three items are measured -- and to show who's great at doing each one.
Pop time

2017 MLB leaders in pop time on steal attempts at second base
1.89 seconds -- Austin Hedges, Padres
1.90 seconds -- J.T. Realmuto, Marlins
1.93 seconds -- Gary Sanchez, Yankees
1.93 seconds -- Martin Maldonado, Angels
1.93 seconds -- Manny Pina, Brewers
Of 53 catchers with 15 tracked throws to second.
MLB average is 2.01. Range is from 1.89 (elite) to 2.14 (poor).
Pop time conventionally refers to the time between the "pop" of a ball hitting the catcher's mitt to the "pop" of it reaching the infielder's glove -- i.e., how fast does he get it from behind the plate to the base. We tried that. It didn't really work, because poor catchers were getting a boost since infielders were coming off the bag to get non-competitive throws in front of the base. Instead, if the throw ends before the base, we account for the projected time it would have taken to reach the center of the base.
As you'd imagine, pop time does matter in preventing stolen bases, though it's also unsurprisingly a pretty noisy relationship. (A pitcher's time to the plate and ability to hold a runner on, the speed of the runner and the accuracy of the catcher's throw are all important factors, of course.)

Based on the data, 0.1 seconds of pop time changes the caught stealing rate by 10 percentage points. Only one of the slowest 10 catchers had a caught-stealing rate of 40 percent. Only one of the top nine -- Hedges -- didn't.
It's also a skill that correlates pretty well year to year. Maldonado, for example, was 1.92 in 2015, then 1.91 in '16 and 1.93 in '17. At the other end, someone like Stephen Vogt was at 2.08 in '15, 2.10 in '16 and 2.11 in '17. It's a combination of two different skills, as you'll see, but we tend to see the same names at the top each season.
Exchange

2017 MLB leaders in exchange on steal attempts
0.64 seconds -- Chris Stewart, Pirates
0.64 seconds -- Salvador Perez, Royals
0.65 seconds -- Yan Gomes, Indians
0.66 seconds -- Welington Castillo, Orioles
0.66 seconds -- Christian Vazquez, Red Sox
0.66 seconds -- Tony Wolters, Rockies
Of 53 catchers with 15 tracked throws to second.
MLB average is 0.73 seconds. Range is from 0.64 (elite) to 0.85 (poor).
Pop time is a combination of two things, really: How fast you can throw it to the base and how fast you can get it out of your mitt in the first place -- i.e. "exchange." They're two very different skills. This is how someone with a relatively average arm like Wolters can have a slightly above-average pop time (1.99 sec), because he can get it out of his glove so fast.
"Usually it comes down to the exchange," Gomes said to MLB.com's Jordan Bastian in 2015.
The Cleveland backstop rated as elite in each of the three years Statcast™ has been tracking this.
"Sometimes that can mess up a fairly good throw," said Gomes. "It can be throws going different ways, but it's usually always about the exchange."
This can help tell some interesting stories. Remember when Jonathan Lester could only throw to David Ross, given his issues with pickoff throws? Part of the reason Ross was so effective was that he had the second-fastest exchange in 2015, and the best in '16.
Arm strength

2017 MLB leaders in max-effort arm strength on steal attempts
87.8 mph -- Gary Sanchez, Yankees
87.7 mph -- Martin Maldonado, Angels
87.3 mph -- J.T. Realmuto, Marlins
87.0 mph -- Willson Contreras, Cubs
86.0 mph -- Andrew Butera, Royals
Of 53 catchers with 15 tracked throws to second.
MLB average is 81.8 mph. Range is from 87.8 mph (elite) to 74.7 mph (poor).
You shouldn't be surprised to see the names here. We've been talking about the great arm of Sanchez since the summer of his rookie year. We've seen Contreras throw runners out from left field as well as from behind the plate, and Butera's arm is respected enough that he's appeared as a pitcher five times in his career. And Maldonado, well, there's little that's more visually appealing in baseball than watching him nail baserunners without getting off his knees.

But it's the name that doesn't appear here -- because he didn't play enough to meet the qualification standards -- who you ought to keep in mind. Philadelphia rookie Jorge Alfaro got into just 29 games and didn't quite make the threshold of 15 tracked throws to second on steal attempts.
If Alfaro had, however, he would have ranked No. 1, with an average of 89.8 mph, because he had three of the top four hardest catcher throws of the season. (Even that comes with a caveat. The hardest throw all year, a 91.1 mph laser from Pittsburgh's Elias Diaz, came from a standing position after he'd jumped in front of the plate to collect a weakly-hit grounder.)

Alfaro had four of the top seven throws, and five of the top 10. He's not on this list yet, but if he gets regular playing time in 2018, you can expect he will be next year. At the other end of the leaderboards, you'll find Tyler Flowers, who had the lowest arm strength (74.4 mph) and slowest pop time (2.14 sec). Of course, Flowers was a strong hitter and an elite pitch framer last year, so the Braves will live with it.
Each of the three new catcher metrics -- pop time, exchange and arm strength -- are available on the Baseball Savant leaderboards, and they will be updated regularly throughout 2018.