The best hitter on the Brewers isn't longtime star Ryan Braun, or 2017's breakout sluggers Eric Thames and Travis Shaw. It's not either of last winter's pair of big-ticket outfield imports, Lorenzo Cain and Christian Yelich. The best hitter on this year's Brewers was cut free a year and a half ago by Cleveland so it could addRichie Shaffer, who never appeared for the Indians and is currently hitting .164/.216/.310 in Triple-A.
This year's best Milwaukee hitter isn't just the best hitter on his own team. Jesus Aguilar, a virtual unknown to most baseball fans a few months ago, has legitimately been one of the most productive hitters in baseball for a first-place team. By hitting .313/.373/.625, he's been as productive asAaron Judge on a rate basis.
2018's best hitters, by wRC+, min. 200 PA
196 -- Michael Trout, Angels
194 -- Mookie Betts, Red Sox
174 -- J.D. Martinez, Red Sox
169 -- Jose Ramirez, Indians
160 -- Aguilar, Brewers
160 -- Judge, Yankees
153 -- Francisco Lindor, Indians
153 -- Freddie Freeman, Braves
(wRC+ is a park-adjusted metric where 100 is league average)
How is that possible? There are really three questions here that we need to answer, in chronological order.
1) Did Cleveland make a mistake in letting him go?
2) What's made Aguilar this productive so far?
3) How likely is this to continue?

1. Should the Indians have regrets?
Sort of! But not really. Aguilar made four Minor League All-Star teams with Cleveland, in 2009 and 2013-15. He hit double-digit home runs six times, topping out with 30 in '16, and he showed good plate discipline, with a walk rate just below 10 percent and a strikeout rate just below 20 percent. The 64 big league plate appearances he received over parts of three seasons weren't exactly enough of a chance for him to show what he could do.
Then again, slow right-handed-hitting first basemen with limited defensive value aren't exactly in high demand in baseball, and it's not like Cleveland was hurting for production from first base and designated hitter, thanks to various combinations of Carlos Santana, Edwin Encarnacion, Mike Napoli, and Ryan Raburn. From 2015-17, Cleveland first basemen were baseball's ninth-best hitters and Cleveland designated hitters were fourth-best.
If the Indians did err, it was in not giving Aguilar more of a chance when they had chances to give. Near the end of an 81-80 season in 2015, they spent far too much time giving DH starts to disappointing veteran Chris Johnson instead of giving younger players like Aguilar a chance. Still, it's not like Aguilar's positions were major weaknesses for good Cleveland teams, either.
(Besides, it's not like Milwaukee did this perfectly either; while it wasn't a one-for-one swap, the roster machinations that got Aguilar a spot headed into 2017 also ended up with the loss of Scooter Gennett, who went on to become a slugging star for the Reds.)
2. What is he doing so well?
Sometimes, this can get pretty complicated. Not so in Aguilar's case, because he's succeeding in the most 2018 way possible: he hits the ball very hard and almost never on the ground.
Of 278 hitters who have had at least 100 batted balls, Aguilar's hard-hit rate of 46.3 percent is ranked 34th, ahead of stars like Matt Carpenter and Bryce Harper.
"The only thing I can control is trying to hit the ball hard," Aguilar told MLB.com in May. "From there, you can't really control what happens."
That's half true, because a hitter also has some control over whether he's hitting it hard in the air or on the ground, and Aguilar is most definitely doing the latter. Of those same 278 hitters, his 30.1 percent ground-ball rate is fifth-lowest.

As you can imagine, hitting the ball hard and in the air leads to elite production, which is how Aguilar has posted a .625 slugging percentage -- fourth in baseball behind Betts, Martinez and Trout (though he's currently a few plate appearances shy of officially qualifying for the league leaders).
But what specifically is he doing to improve over last year's solid-but-unspectacular .265/.331/.505 season? Look to the fastballs.
Last year, Aguilar hit .242/.345/.474 with a 33-percent strikeout rate against fastballs, which was fine enough. This year, he's managed to pull off a neat trick: he's swinging at more of them, up to 45 percent from 43 percent, and making a ton more contact in the zone, up from 74 percent to 88 percent, which is a huge jump.
If you look at the heat maps, it's easy to see what's happening. Last year, he would go after the high heat, with limited success. (He hit .167 with a .333 slugging in the upper third of the zone against four-seam fastballs.) This year, he's laying off the high fastballs that he can't do much with.

Instead, he's focusing on the outside of the plate, and that's a great spot for him -- he's slugging .931 on the outer third against fastballs of any type. That's not just a good number, it's the third-highest in baseball.
3) Is this at all sustainable?
The first thing you think of when a player with limited track record comes up and starts to mash is that he's possibly the beneficiary of fortunate luck -- that balls are finding holes. We saw this with Tampa Bay's Mallex Smith in April, when he hit .329/.391/.418 thanks to an unsustainable .406 BABIP. With a more reasonable .317 BABIP since May 1, he's hit just .241/.306/.337 since.
We're not going to guarantee anything with Aguilar, because baseball doesn't work that way. We can, however, say that he's earned just about all of his production, at least based on Statcast™ metrics.
The way we do that is to look at player's actual production and compare it to their expected production, based on the underlying skills of exit velocity, launch angle, strikeouts and walks. (Another way to think about it is that if a batter squares up a ball in a way that's a hit 80 percent of the time but gets an 0-for-1 due to a spectacular defensive play, we want to give him credit for hitting the ball in such a productive way.)
Looking at it that way, for example, we can say that Albert Almora Jr.may be the player most outperforming his skill, since he's putting up a great line without any meaningful changes to his strikeouts, walks, exit velocity or launch angle, and that could be an indicator his performance may not last.
That's not the case for Aguilar, however. Last year, his actual wOBA was .351 and his expected was .346 -- basically identical. This year, his actual wOBA is .411, and his expected is .403 -- or again, more or less identical.
That doesn't guarantee anything going forward. It does suggest that what we've seen so far hasn't been about good luck. Aguilar hits the ball hard, he hits it in the air and he's got a nice Minor League track record to fall back upon. For someone who entered the spring uncertain of a job with Braun expected to be the right-handed-hitting half of a platoon with Thames, Aguilar has produced a fantastic season -- and from this view, there's little reason to think it can't continue.