A case for Realmuto as MLB's best catcher

Last year, we crowned Miami's J.T. Realmuto as baseball's "most athletic catcher," thanks to a combination of Statcast-based metrics that showcased his speed on the bases and skill behind the plate.
A year later, it's becoming clear that title might not have been enough. What if Realmuto is simply 2018's best all-around catcher? He may not have the name recognition of Buster Posey or Yadier Molina, but it's actually an easier case to make than you might think. Let's count down all the things Realmuto is shining at, shall we?
It matters that Realmuto has been baseball's best hitting catcher this year (he has) and that he's baseball's fastest catcher (he is), and we'll get to those. Let's start with what's happening behind the plate, about how he's been cutting down opposing baserunners, since that's what people think of when they think of catchers. Let's start with pop time.
Realmuto has the fastest pop time and the best caught-stealing percentage.
There's a lot that goes into being a good catcher. Some of it we can quantify very well, and some of it, like calling pitches or handling pitchers, we can't. (Realmuto is "the leader of the team ... the leader of the staff, the hitters," said Marlins reliever Kyle Barraclough. "Not much more I can say.")
One of the things we can measure very well is pop time, which is simultaneously very new and extremely traditional. It's only been available publicly via Statcast™ leaderboards for about a year, but it's something scouts have been hand measuring with stopwatches for decades. It's intended to express how quickly a catcher gets the ball out of his glove (referring to the "pop" of the pitch hitting his mitt) to the infielder receiving the throw on a steal attempt (another "pop" of the throw reaching the fielder), though it's technically measured to the midpoint of the intended base.
It's a combination of two things, really: "How fast can you get the ball out of your glove?" which we call "exchange"; and "How fast can you throw it to the base?" -- which we express as "arm strength." This doesn't capture every facet of preventing steals (like throw accuracy, for example), but it's a good way to measure skills.
Among catchers who have faced five steal attempts of second base, Realmuto has 2018's fastest pop time, at 1.86 seconds, well below the Major League average of 2.01 seconds. While he's been above average from the day he arrived in the big leagues, he's actually gotten faster, dropping from 1.92 seconds in 2015 to 1.91 in '16 to 1.90 last year, then to 1.86 this season. Since '15, Realmuto is tied with Austin Hedges for first among those with 50 steal attempts, at 1.91. He's been in the top two every year.

Of catchers with at least 20 steals attempted against them, Realmuto has the best caught stealing percentage at 44 percent. We know that a catcher is not entirely responsible for that -- a pitcher's ability, or lack thereof, in holding a runner on plays a big part -- but we also learned last year that 0.1 seconds of pop time changes the caught stealing rate by 10 percentage points, so it matters.
So how does a catcher get to be good in pop time? You can have a cannon of an arm like the Phillies' Jorge Alfaro, who leads the Majors with a 90.4-mph average on his throws. You can have an elite exchange time like Welington Castillo of the White Sox, who got rid of the ball in just six-tenths of a second. Or you can be Realmuto, who's good at both. He's got the third-best arm strength, 87.6 mph, behind only Alfaro and Martín Maldonado. He's got the third-best exchange time too, at 0.68 seconds, behind Castillo and Carlos Pérez.
Thanks to that combination, Realmuto has three of the five fastest individual pop times to catch a runner at second base this year, led by this 1.76-second laser to catch Amed Rosario on May 23.

Realmuto has been baseball's best hitting catcher.
In 2016-17, Realmuto (.290/.337/.440) was the fifth-best hitting catcher among the 17 who had at least 750 plate appearances. This year (.300/.358/.531), he's been the best of the 14 catchers with at least 200 plate appearances so far. It's not that Realmuto has changed his strikeout or walk rates, because they've stayed similar to his past years. It's that he's hitting the ball much harder, and he's hitting it off the ground.
Realmuto's hard-hit rate has hovered in the 34-35 percent range for each of the past three years. This year, that's up over 43 percent, similar to Francisco Lindor or José Ramírez. He's also cut his ground-ball rate from 49 percent to 43 percent, allowing him to take more advantage of that power.

We can't guarantee that Realmuto will outhit Posey, Gary Sánchez, Yasmani Grandal and the rest all year. But he has so far.
Realmuto is baseball's fastest catcher.
"I've always been fast, from playing football, basketball," Realmuto said last year. "It didn't matter, whatever sport I was playing, I was always running. I tried to do my best to keep my athleticism with where it's at."
We have four years of data in Statcast™'s speed metric, Sprint Speed, and Realmuto has been the fastest catcher every year. This year, his average top speed has been faster than Yasiel Puig, Mookie Betts or Albert Almora Jr. While he stole 20 bases over the past two years, this speaks more to his overall athleticism than anything else.
Realmuto may be baseball's best all-around catcher.
Putting it all together, Realmuto leads all catchers in two very different versions of Wins Above Replacement. He's No. 1 at FanGraphs, just ahead of Francisco Cervelli, in a version of WAR that does not account for pitch framing. Realmuto is No. 1 at Baseball Prospectus, just ahead of Grandal, in a version that does. (While he's never been considered an elite framer, he was solidly above average last year, 13th, at plus-nine runs.)
So is Realmuto baseball's best catcher? It helps him that it's been a down year for backstops, that Salvador Perez, Molina and Tyler Flowers have been injured, that Sanchez has struggled, that Posey is now 31 and Russell Martin is 35.
But Realmuto will almost certainly represent the Marlins in the All-Star Game, and he's got a strong case to be the National League's starter, even though he's unlikely to win the vote. He's a pop time star, and he's good at almost everything. Realmuto is almost certainly the best catcher that not enough people know about.