Scouts spend countless hours watching and evaluating players, carefully considering the appropriate grade for each tool or each pitch a player offers. Throughout the course of the season and particularly throughout the course of ranking season, grades are tossed around with near reckless abandon. This player has plus power, and that player has a below-average fastball. This player offers above-average hit projection while that player buries hitters with a potential plus-plus curveball. It's easy to talk about the quality of an individual tool, but what does it all mean in the context of other players?

In the second edition of the annual Top Tools Series, the Baseball Prospectus Prospect Staff debated long and hard over how individual players’ tools stack up against those of their counterparts. Drawing upon our own eyewitness accounts and opinions from scouts across the league, the team debated and compiled the following ratings. The end result is a product that captures the oft-missing context of how individual player tools compare and who has the best of each tool in baseball.

Previous entries: Best Hit/Power | Best Speed/Makeup | Best Infield Defense/Infield Arm

Outfield Defense

Top Outfield Defense in the Minor Leagues: Kevin Kiermaier (Tampa Bay Rays)
Though Kiermaier wasn’t the unanimous winner in this category, his defense earned universal praise across the scouting community. On the back of plus-plus speed, Kiermaier shows off tremendous instincts for the position. His reads off the bat are lightning quick and his routes are nearly impeccable, allowing him to get to balls deep in the gaps as well as those in front of him and over his head. He doesn’t consistently struggle with movements in any direction and profiles as one of the best outfield defenders in the majors. He should be a true difference maker with the glove, even if his role consists of coming off the bench.

Others Considered: Albert Almora (Chicago Cubs), Jackie Bradley (Boston Red Sox)
Jackie Bradley earned heaps of praise from scouts and was nearly considered the co-winner with Kiermaier. Bradley’s speed doesn’t play as big a role as Kiermaier’s, with Bradley playing the part of an average runner who relies more on instinct and effort to make plays in center field. In spite of that, Bradley is considered one of the best outfield defenders in the game and a likely Gold Glove candidate at his peak. Though Almora has plus defensive projections, he rates a solid step behind the likes of Kiermaier and Bradley, simply because his speed may prevent him from sustaining a plus profile in center field. A fringe-average runner, Almora has exceptional reads and routes that allow him to make plays in center, but there are lingering questions about whether he can maintain his current defensive ratings as he matures and settles into a big-league role.

Top Major League Outfield Defense: Peter Bourjos (St. Louis Cardinals)

All-Time Tool: Devon White

How to Identify It: Simply put, good defenders look like naturals. The most important aspect of outfield defense is reaction time. The best defender will naturally react to the ball off the bat, making the correct movement and using the correct path toward the ball. He will often make it look easy, even though some other players might have needed to dive for the same ball. Footwork is important, and one wrong step backward or forward can often spell the end of any chance an outfielder has to make a play on the ball.

The path to the ball is probably the second-most-important aspect of defense. A straight line is always the fastest route, and good defenders will be able to map out the end point of the ball. Once a player starts looping or running off the straight-line path, he is losing time to set up for a potential throw. Good defenders are able to situate themselves in a manner that is necessary for post-play maneuvers. Defense at and around the wall is often overlooked in casual analysis of defensive play. A good defender is able to have that sixth sense of where the warning track and wall are, and he is able to time his jump, if necessary. The bottom line is that reaction and judgment skills are extremely important for any defender in the outfield, especially in center. —Tucker Blair

Outfield Arm

Top Outfield Arm in the Minor Leagues: Tyler Naquin (Cleveland Indians)
While opinions varied on the strongest arm in the minor leagues, Naquin certainly had his share of supporters. Seeing Naquin myself last year, he had an easily recognizable plus-plus cannon attached to his body, with the ability to unleash throws from the deepest parts of the park. Aside from plenty of natural velocity, Naquin’s throws carry extremely well, and he displays very good accuracy when throwing to any base. Naquin’s arm strength fits well in right field, but his defensive chops have improved enough for him to stick in center, making his arm an absolute weapon.

Others Considered: Jorge Bonifacio (Kansas City Royals), Che-Hsuan Lin (Houston Astros), Hunter Renfroe (San Diego Padres), Jorge Soler (Chicago Cubs), Bubba Starling (Kansas City Royals)
Hunter Renfroe received serious consideration for the top spot on this list, with strong support coming from several amateur scouting directors who were adamant regarding the premium strength of his arm. Lin has had trouble pinning down a major-league job, but that is not a reflection of his plus-plus arm that can make baserunners think twice before taking an extra base. Scouts that cover the Royals spoke very highly of both Bonifacio and Starling, with many giving them 6+ to 7 grades on a consistent basis—though both were held back a bit by some reports of inconsistency with the quality of their throws. Cubs right field prospect Jorge Soler was also plagued by inconsistency on this throws in 2013, but his raw arm strength stands with that of every player on this list, and he has the potential to top the list in the future.

Top Major League Outfield Arm: Bryce Harper (Washington Nationals)

All-Time Tool: Jesse Barfield

How to Identify It: Arm strength is a simple tool to evaluate in comparison to others. More often than not, your eyes will not be deceived by what is displayed on a throw. Of course, there are a few things that should be looked for in a throw from the outfield. Just as with pitchers, the arm speed is usually a strong indicator of the amount of force that follows a throw. I also make a point to look at the angle of the arm, as some throws will tend to drift horizontally if the throw is not coming over the top. While movement is a plus for a pitcher, in the outfield it leads to a headache for those on the relay. Tailspin is bad and is often the reason for trouble on a throw.

Some throws also tend to break through the “second level” of wind. What I mean by this is that they seem to cut through the air and don't fade on their path to the cutoff man. The plus and plus-plus arms will routinely be able to hit the cutoff man on a line and even reach home plate on a line. Finally, I look at the crow hop and the footwork involved. While the feet aren't directly responsible for the throw, they do provide some of the force required, and a good crow hop will allow the player to gain extra momentum. A productive crow hop combined with the correct arm angle and arm speed allows a player to produce a terrific throw from the deepest part of the park. —Tucker Blair

Article discussed and debated by the Baseball Prospectus Prospect Staff. Constructed and delivered by Mark Anderson.

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Lin is, reportedly, being converted to pitching.
Josh Reddick should have at least been included as "Others Considered" for Best Outfield Arm.
The "Others Considered" category only appliees to the minor leagues. Across the entire series only one MLB name is listed.
Devon White? Really? Maybe most fluid and fun to watch when he was on his game. But I saw a lot of games in Toronto where he was on some other planet, usually when Joe Carter wasn't playing right field. The joke in Toronto during those years was that White didn't play for the Jays, he played for Joe Carter.

I never heard that joke, and I caught about 500 Jays games between 91 and 95. You may be the first Jays fan I've heard of who wasn't impressed with Devo's defence.

I am sure we can both agree on Barfield's arm, though. This was from his Yankee days, gunning down Chili Davis:
Vlad Guerrero and of course Bo Jackson probably also in the all-time arm category.
adrock when I first read your comment- you "caught 500 Jays games"- I thought you literally meant you were a catcher for the Blue Jays.

--runs to b-ref Blue Jays franchise encyclopedia--
--deduces adrock must be either Pat Borders or Randy Knorr--
--re-reads comment--
I just want to make sure the name of Willie Mays gets mentioned in this thread.
I knew Devon White was a pretty good defender (BBREF sees him as above average until he got into his early 30's) but had no idea anybody thought of him as one of the best defensive outfield prospect of all-time.
I have trouble with the idea of putting him above Andruw Jones. Perhaps a carer value over peak value consideration? Jones did fade pretty hard once he hit 30
I love the shout out to Devon White. He triple play that wasn't in the WS was one of the all-time great catches.
Silly to argue about who the all time greatest was at anything...but I'll take one of the Garys - Pettis or Maddox.
I would also add that I'm shocked to see Harper as best contemporary arm. Certainly one of the strongest, but I doubt he makes my top 5. There was a time when Ankiel played next to him in CF where he didn't have the top arm in his own OF.
Were Gerardo Parra or Juan Lagares even mentioned or brought up in the discussion for best outfield arm?
GCFOAT=Andruw Jones?

Best arm in the majors is probably Jeff Francoeur if he counts. If not, Alex Gordon's gotta be close.
Since all these names are being thrown around, how about Richie Ashburn. He has five of the top ten seasons in putouts for a CF, led the league nine times, 2nd all-time in 15 seasons played.
Best outfield arm ever was Roberto Clemente.
+1 to andruw. Don't really think it's close either. His uzr was insane. Sick he tossed in a 51 HR season too.
Best outfield arm ever: 1) Al Kaline 2) Roberto Clemente
Mark Whiten had the strongest arm I've ever seen.
It's Roberto Clemente. I have seen everyone listed here throw. Clemente could out-throw all of them.

Here's a good discussion about why it's Clemente:
Bill James: "I’ve been trying a little experiment, asking baseball fans that I meet who had the best throwing arm they ever saw. It’s very rare that anybody who is old enough to remember seeing him play doesn’t immediately say “Clemente.” For younger fans, you just can’t believe what it was like; I hope we see another one like it, or you’ll never believe that it was possible. His throws combined strength, accuracy and speed of release in whatever proportions were necessary to get the job done."