Let's not bury the lede here, and instead get right down to business: The Houston Astros have baseball's best outfield throwing arms, by a lot. As a group, they lead the game by nearly a full mile per hour over second-place Baltimore. As individuals, the Astros' four primary outfielders all appear in the top 45 of a group of more than 120, including each of the top two spots. At 94.6 mph, their average power on "competitive throws" -- we'll explain in a moment -- is a full 10 mph more than the 84.6 mph put up by the 30th-place Giants.
We need to make sure that's known right up front, because it's surprising how much explanation needs to go into something that seems as simple as measuring throwing velocity. The best thing about Statcast™ is that it measures everything, the worst thing about Statcast™ is that it measures everything. For example, Mike Trout had a few hundred tracked throws in 2015. But many of them weren't really relevant; to pick one at random, on June 1 against the Rays, he collected a sixth-inning Rene Rivera single with the bases empty and lobbed it back in at 44.6 mph. Think about how many times that happens over the course of a season.
So to overcome that, we had to come up with a way to track only serious, competitive throws. Instead of putting a one-size-fits-all minimum threshold, we decided to individualize it a little better. Here's what we went with:
1. Identify a player's 90th percentile arm strength.
2. Take the average of all throws above the 90th percentile.
When we set the minimum number of qualified throws at 45, we're left with 121 outfielders, which works out nicely to be roughly three starters and a backup per team, though it didn't always end up that way. (Outfielders are presented here with their 2016 clubs, so Jason Heyward is a Cub, Justin Upton is a Tiger, and so on.)
So, back to the Astros: As you can see, their group of competitive throws -- nearly all by Carlos Gomez, Jake Marisnick, George Springer, Colby Rasmus and Preston Tucker -- made for the only team to top 94 mph, edging out the Orioles and White Sox in second and third place:

The majority of teams occupy a band between roughly 89 mph and 92 mph; the fourth-place Angels are as close to the Astros as they are to the No. 15 Cubs. It's even more impressive than that for Houston, though, because the Astros also swept the top two spots on the individual rankings:
Top outfield arms on "competitive throws," minimum 45 qualified throws

1. Marisnick 98.6 mph
   
2. Gomez, 98.3 mph
   
3. Kevin Kiermaier 97.9 mph
   
4. Avisail Garcia 97.0 mph
   
5. Carlos Gonzalez 97.0 mph
   That's right -- another thing Kiermaier is great at, though it should be pointed out that Marisnick's average here was topped by only one pitcher all season: Aroldis Chapman. Probably heard of him. Throws hard. Gomez, in fact, had the hardest throw by any outfielder all season, touching 103.1 mph on this laser to nail Joe Mauer at the plate in September:
   Gif: GIF HOU Gomez Thows out Mauer 9415
   Perhaps now you're thinking that the hardest throws of the year were all with considerable forward momentum, an outfielder slowly coming forward to make the catch and helping to push his throw towards the plate. Sure, some of them were, but not, as Aaron Hicks showed with this flat-footed jumping catch in Kansas City, always:
   Gif: Aaron Hicks throw
   Hicks' 102 mph throw wasn't close to being on target, which is a reminder that arm strength and arm accuracy aren't always the same thing. (On that note, none of Marisnick's three throws tracked above 100 mph resulted in an assist.)
   For context, let's show the end of that list of outfielders with at least 45 "competitive throws."
   117. Michael Bourn 81.2 mph
   
6. Corey Dickerson 81.7 mph
   
7. Dustin Ackley 81.5 mph
   
8. Cole Gillespie 81.2 mph
   
9. Khris Davis 78.1 mph
   This is shown not to put down any of these players so much to show that the methodology passes the smell test. Ackley looks likely to be primarily an infielder this year, and Davis, traded recently to Oakland, has been limited to left field largely due to his arm, despite the outstanding power in his bat -- he has only four assists in nearly 2,300 innings of outfield play over the past three seasons.
   As for the Giants? Angel Pagan (112th, 83.2 mph) and Gregor Blanco (113th, 82.7 mph) were joined this offseason by Denard Span, a solid player who made plenty of sense but isn't known for a strong arm, as he ranked 103rd here, at 85 mph. (Hunter Pence made a stronger showing, coming in 81st at 87.8 mph.)
   What we have here is something of a 2016 outfield arm strength preview, given the fact that we've put old names on new teams, though we're not accounting for likely newcomers such as Rymer Liriano or Byron Buxton. What we have here is some statistical backing to something we've been trying to put the eyeball test to for generations. What we especially have, is evidence that you might not want to risk that extra base if you're playing Houston. Those Astros outfielders sure can throw.