Fangraphs.com has an interesting statistic called run value by pitch type, which is explained here and here. This raises the fascinating question, which Astros' pitchers are the most effective at throwing various types of pitches? This exercise probably is best saved for the end of the season, because of the sample size issues. But, heck, this is just for fun. Let's see what kind of results we can get right now.
The run value statistic examines the outcome of each type of pitch and the count in which it occurred. The run expectancy varies depending on the count; for example, the liklihood of a good offensive outcome is much different for 3-0 than it is for 0-2. A positive number indicates "runs saved above average" for the pitch. I will use the rate version of the stat which reflects runs saved above average per 100 pitches.
I excluded Valverde from this comparison, because he hasn't pitched enough to show anything meaningful. After some debate, I decided to show the results for relief pitchers and starting pitchers separately for some of the pitch types. Clearly the two types of pitchers produce different ranges, and I'll discuss that shortly. Since I'm only concerned with the best pitches, and not the worst ones, I leave out any consideration of pitches with negative runs.
BEST FASTBALL
(R) Fulchino +2.47 Arias +1.5
(S) Wandy +.57 Ortiz +.51
BEST SLIDER
(R) Arias +9.56 Sampson +3.46
(S) Ortiz +.68 Oswalt +.19
BEST CURVE
Wright+ 4.18 Wandy +1.47
BEST CHANGE UP
(R) Sampson +7.64 Fulchino +5.07
(S) Hampton +2.25
Another characteristic of interest is the percentage of swings outside the strike zone induced by the pitcher. This might tell us something about pitchers with nasty stuff.
HIGHEST PERCENT SWINGS AT BALLS OUTSIDE THE ZONE
Arias 30.1%
Wright 27.4%
Wandy, Hampton (tied) 25.8%
Two young guys who have surprisingly impressive pitches: Fulchino and Arias. Chris Sampson usually had the reputation as a guy with fringe type stuff. But Sampson has great success with some of his off-speed pitches. Something I didn't show above is that Sampson has had excellent success with his cutter, even though he doesn't throw it very often.
And the guys mentioned above are all relief pitchers. Clearly relief pitchers seem to be capable of putting up higher "runs saved" numbers on pitches, compared to starting pitchers. I think this partially reflects sample size, both in terms of innings pitched and the tendency to throw the "out" pitch a small percentage of the time. Relief pitchers usually don't face the hitters more than once, which gives them an advantage over starters (and one of the reasons that relief pitchers have lower ERAs on average than starters). Also, relief pitchers may "surprise" hitters more when they throw a specific pitch sparingly. For example, when a reliever saves a pitch only for the most critical count (say, 3-2) the runs value will be better than a starter who throws his best pitch in a variety of different counts. While Wesley Wright's curveball looks pretty good, I doubt that it is better than Wandy's, as the numbers above might suggest. An important point is that Wandy throws the curveball a bunch more than Wright. Wandy throws his curveball 33% of the time; when you think about how often he throws it, the 1.47 runs saved above average rate is remarkably good.
You also have to resist the temptation to use these stats to argue that a particular pitcher should throw a high value pitch more often. The value of the pitch may depend on when and how the pitcher uses it. For example, the sequence in which fastballs and sliders or curves or mixed into the count probably is important to the result.
I wonder how these numbers will change over the course of the season.