The fun thing about trying to identify 2017's most effective pitches is that there's a million different ways to do it. Lowest batting average? Sure. Most strikeouts? Fine. But what we prefer to use around the Statcast™ lab is a metric that accounts for all the things a pitcher is trying to do. When a pitcher releases a pitch, he wants to get strikes, limit walks and induce contact with low likelihoods of success, either low exit velocity or high exit velocity at a bad angle.
That's what we use Expected Weighted On-Base Average for, and it's a lot simpler than it sounds. It accounts for the expected outcome of each batted ball, strikeouts and walks. It combines the amount of contact with the quality of contact, and it puts it on a scale similar to regular OBP, except that extra-base hits are given more value.
Not only that, we're eliminating the effects of defense and ballpark, focusing only on the moment of contact. If a pitcher gets a weak popup that a poor outfielder turns into a double, let's credit him for the weak contact. If he allows a screaming liner that a great fielder saves for an out, let's credit him for the liner.
With that introduction out of the way, let's get to the main event: Which starters and relievers had the most effective pitches of 2017? Setting a minimum of 200 pitches, we can look at the primary pitch types and find some interesting names -- both expected and surprising.
Four-seam fastball

Starter
.221 -- Charlie Morton, Astros
.234 -- Chris Sale, Red Sox
.247 -- Brandon McCarthy, Dodgers
.256 -- Jacob deGrom, Mets
.265 -- Rich Hill, Dodgers
Major League average -- .351
Sale is a stud, and so is deGrom, and Hill has been a successful starter for a few seasons, surviving almost entirely on two pitches, so it's not surprising to see any of them on this list. Maybe after all the press he got in the World Series, even Morton's ranking isn't stunning, given that he stayed healthy and upped his velocity from the 91-92 mph it had long been to a career-high 95.7. But McCarthy, the oft-injured Dodger traded to Atlanta? He allowed just a .194 average against the fastball. It's a better pitch than you think.
Relievers
.181 -- Ryan Madson, Nationals/Athletics
.205 -- Anthony Swarzak, Brewers/White Sox
.207 -- Brandon Morrow, Dodgers
.210 -- Chad Green, Yankees
.216 -- Brad Boxberger, Rays
Major League average -- .335
Note that of the five relievers on this list, three found new homes this winter. Swarzak signed with the Mets, Morrow with the Cubs, and Boxberger went to Arizona. Meanwhile, Green may have been the biggest pitching breakout story of the year for the Yankees, who moved him to relief and watched him become a star.
Two-seam fastball/sinker

Starters
.284 -- Brad Peacock, Astros
.284 -- Alec Asher, Orioles
.285 -- Jaime Garcia, Yankees/Twins/Braves
.290 -- Rafael Montero, Mets
.291 -- J.A. Happ, Blue Jays
Major League average -- .359
Relievers
.242 -- Matt Albers, Nationals
.246 -- Richard Bleier, Orioles
.248 -- Matt Bowman, Cardinals
.250 -- Pat Neshek, Rockies/Phillies
.251 -- Bud Norris, Angels
Major League average -- .341
We'll take both of these together, because the story for both starters and relievers is similar. You know about Peacock, who starred in the World Series; you should know about Neshek, who just returned to the Phillies. But maybe you didn't know about Bleier, who had a 1.99 ERA and the second-highest ground-ball rate of any reliever, and it might have been easy to miss the fact that Albers, currently a free agent, had a 1.62 ERA for Washington in 2017. This, in part, is how.
Cutter/Slider

Starters
.141 -- Robert Stephenson, Reds
.149 -- Luis Castillo, Reds
.152 -- Carlos Carrasco, Indians
.164 -- Max Scherzer, Nationals
.185 -- Sonny Gray, Yankees/Athletics
Major League average -- .272
Relievers
.138 -- James Pazos, Mariners
.139 -- Felipe Rivero, Pirates
.140 -- Raisel Iglesias, Reds
.154 -- Arodys Vizcaino, Braves
.154 -- Kyle Barraclough, Marlins
Major League average -- .253
Other than more reaffirmation that "yes, Scherzer is a legend," please direct your attention to two areas of this list. First, Rivero is the most dominating pitcher who never seems to get the respect he deserves; while the trade that shipped out Mark Melancon was unpopular in Pittsburgh in 2016, it now looks like an absolute steal after the flame-throwing Rivero whiffed 88 in 75 1/3 innings in 2017.
Second: It appears the Reds might have themselves a type. Castillo, Stephenson and Iglesias are all young, talented arms, and they each appear to excel in one of these two similar pitch types. 
Curveballs

Starters
.129 -- Michael Wacha, Cardinals
.133 -- Corey Kluber, Indians
.159 -- Blake Snell, Rays
.169 -- Tyler Chatwood, Rockies
.171 -- James Paxton, Mariners
Major League average -- .257
Relievers
.111 -- Will Harris, Astros
.133 -- Keone Kela, Rangers
.138 -- Thomas Pressly, Twins
.140 -- C.J. Edwards, Cubs
.148 -- Cody Allen, Indians
Major League average -- .239
It's a never-ending discussion as far as what to call Kluber's breaking pitch -- "It's a slider, curve and cutter all in one," laughed teammate Josh Tomlin when asked in October -- but here it's a curveball, because that's what Kluber calls it, and it is, of course, stellar. You knew that, as you knew that his Cleveland teammate Allen is a star. They're not the only interesting names here, though. We gave five big reasons that Chatwood could have a breakout season before he signed with the Cubs, and both Pressly and Chatwood are in the elite level of the curveball spin leaderboards.
Changeup

Starters
.160 -- Stephen Strasburg, Nationals
.175 -- Castillo, Reds
.199 -- Carrasco, Indians
.204 -- Luke Weaver, Cardinals
.208 -- Scherzer, Nationals
Major League average -- .302
Relievers
.166 -- Tommy Kahnle, Yankees/White Sox
.185 -- Fernando Rodney, D-backs
.192 -- Trevor Hildenberger, Twins
.199 -- Chris Devenski, Astros
.203 -- Rivero, Pirates
Major League average -- .273
There's Scherzer again, and Castillo, and Carrasco, which is evidence that very good starters generally have more than one good pitch. We've long known that Devenski's changeup has been his not-so-secret weapon, and it's almost unfair that Kahnle, who averages 97.8 mph on his fastball, could have a changeup this good. Then again, that's almost certainly not unrelated. A good fastball and a good changeup often go hand-in-hand.