There's no shortage of dominant relievers in baseball these days, but for all the greatness we're seeing from stars like Andrew Miller, Greg Holland, and Wade Davis, plus lesser-known names like Felipe Rivero and Corey Knebel, there's two studs pretty clearly standing head and shoulders above everyone else: Craig Kimbrel and Kenley Jansen. They've been baseball's two best relievers this year, and they've been two of the best for the last half-decade, too.
But which one has been better in 2017? The right answer, as it usually is, is that you could make a case for either one. They're both elite. They're both practically unhittable. They're both having the best seasons of what were already great careers, and they both may very well end up in Cooperstown next to one another someday. They're that good.
Still, there's one thing that so far sets Jansen apart -- and it's enough to call him baseball's most dominant reliever. (Your mileage may vary.) 

Before we can get into how either one stands apart, we have to explain the ways in which they're the same, because there's really so many. Entering Thursday's games, they've faced nearly the same number of hitters (111 for Kimbrel, 107 for Jansen), and their ERA marks are essentially identical (0.85 for Kimbrel, 0.91 for Jansen, and no, a difference of 0.06 doesn't matter). Looking at FIP, an ERA-like estimator based on strikeouts, walks, and home runs, Jansen's 0.30 and Kimbrel's 0.38 are, again, basically identical.
They've each allowed one home run. They've each allowed three earned runs. They've both hit one batter. Kimbrel has more saves, but we all know that saves don't really matter -- four of the last five Dodgers wins have been by four runs or more. If you still think saves are important, then you must think that Minnesota's Brandon Kintzler (19 saves) has been a better closer than Jansen (15).
While Jansen has notably set, and keeps expanding, a Major League record by striking out 50 without a walk, Kimbrel has struck out 59, giving him a superior 53.2 percent strikeout rate to Jansen's still-absurd 46.7 percent. But since Kimbrel has walked five, the difference between his strikeout rate and his walk rate is 48.7 percentage points. The difference between Jansen's is 46.7 points. Again, essentially identical.

Obviously, neither one allows a whole lot of contact, and as you can tell, we're splitting hairs here to say either is better than the other, since they're both unbelievable. That said, there is one area where Jansen pulls ahead: quality of contact.
The simple way to show that is to look at average exit velocity, where Jansen's average of 82.3 mph against is well better than the Major League average of 86.9 mph, while Kimbrel's 89.7 mph is higher than the average. But we can do better than that, can't we? We know that the combination of exit velocity and launch angle is what really matters, and we know that we can look at the historical outcomes of any given batted ball to see how likely it was that a pitcher just allowed a hit. Whether the ball actually does land for a hit often depends on defense, luck, or factors a pitcher can't control.
For example, when Jansen got Jett Bandy to weakly fly out to end a game in Milwaukee earlier this month, we know that's the type of ball which falls for a hit just 3 percent of the time. If outfielder Chris Taylor had botched it and the ball fell for a double and a run, it would have hurt Jansen in the box score, but it doesn't change the skill he showed in inducing such a low-value batted ball. That's what we credit him for.
We can take that a step further, too, because simply looking at "a hit," as batting average does, doesn't tell you about the quality of that hit. Allowing a home run is worse than allowing a single, right? Of course it is, which is why we prefer to use wOBA, or Weighted On-Base Average, since that gives proper credit to more valuable types of getting on base. For reference, the 2017 Major League wOBA average is .321.
So, all that being said, if we were to look at the 440 pitchers who have faced at least 75 hitters this year, sorted by lowest expected wOBA (based on strikeouts, walks, and quality of contact), you won't be surprised at the first two names. You might, however, be surprised at the gap between them.

Lowest Expected wOBA, 2017 (min. 75 pitchers)
.164 -- Jansen, Dodgers
.194 -- Kimbrel, Red Sox
.205 -- Jose Leclerc, Rangers
.210 -- Dellin Betances, Yankees
.211 -- Matt Albers, Nationals
How do we find such a difference, particularly despite the fact that Kimbrel's actual wOBA allowed of .128 is lower than Jansen's actual mark of .169? Because Jansen has induced weaker contact, but despite that, he's found some worse outcomes.
Here's what we mean, looking down the list of each of their batted balls along with expected outcomes. The most dangerous batted ball Kimbrel allowed was hit at 106.2 mph by Kendrys Morales and went for a homer; the most dangerous ball Jansen allowed was hit at 104.2 mph by Justin Bour and went for a homer. Nothing unexpected here.
But here's where things differ. Kimbrel allowed the next four highest expected value batted balls, and only one went for a hit, a Josh Bell double on April 3. For that, you can thank Jackie Bradley Jr., and you can thank Fenway Park.
Here's the second-most dangerous ball Kimbrel allowed, 102.6 mph to Ryon Healy. This type of ball is a home run 70 percent of the time. Bradley was not having it:

Here's the fourth-most dangerous ball Kimbrel allowed, 101.3 mph to Mark Trumbo. This ball is a hit nearly 80 percent of the time, and it's a homer just over half the time. But fortunately for Kimbrel, balls that travel 390 feet don't make it out of center field in Fenway Park.

Here's the fifth-most dangerous ball Kimbrel allowed, 106.5 mph to Manny Machado. This ball was too low to ever be a home run, but it's a hit more than 70 percent of the time… at least in most parks. 

Only one of the 11 most dangerous balls Jansen allowed became an out, while Kimbrel got outs on six of his most dangerous 11. That's a combination of fortunate luck and the fact that Bradley and Mookie Betts are far better defenders than whichever combination of Joc Pederson, Yasiel Puig, Taylor, and Enrique Hernandez the Dodgers are rolling out on any given day.
Both, again, are elite. Both will be All-Stars; each might get some down-ballot Cy Young support. If you prefer one to the other, you won't be wrong. But as we know, outcomes invite other factors. In terms of skill in preventing dangerous contact, of avoiding loud batted balls, Jansen is edging out Kimbrel so far -- if only by a hair.