For the past few seasons, the best defenders in the NBA have been some combination of Draymond Green, Rudy Gobert and Kawhi Leonard. But with Leonard and Gobert out for all or much of the season so far and Green’s production falling off just a bit, we’ve got some new blood in contention to be the NBA’s best defender.To put a number to this, we’ll use data from Second Spectrum that shows us both the total number of shots defended by a player and how much he affected those shots. There are a few ways to get at this, but we’re going to use “quantified shot probability” (qSP) — which determines the expected value of shots defended using shot distance, shot location and defender position, among other variables — and “quantified shot making” (qSM) — which subtracts the expected effective field goal percentage on those shots from the actual eFG to determine how much of an effect the defender had. In other words, does the defender make shots worse? And if so, by how much?We’ve used metrics like this in the past, but the rub with this set is it adjusts for who’s taking the shot, meaning weak defenders who guard bad shooters don’t get as much credit, and defenders tasked with guarding superstars aren’t punished for their assignment — the star’s excellence is baked into the stat.The per possession numbers will look a little different from the overall effect — players like Tarik Black and Marreese Speights have defended a lot of shots very well in short minutes — but this chart should demonstrate that there is a wide spread on defender effect. What we’re missing here, though, is how many shots a defender actually affects. Kevin Durant, Golden State WarriorsThe Warriors are unfair. Draymond Green — for my money the best defensive player in the league — has for whatever reason been a little off during the first quarter of the season, possibly because the Warriors can sleepwalk to a bare-minimum 2-seed. He’s not been bad, mind you, just not quite as dominant as usual. That’s left quite a lot of slack in the Golden State scheme, and Durant has picked up more than his fair share of it. Golden State ranks eighth in defensive rating, down from second a season ago, but that almost feels like a threat more than a falloff given how flat the team has looked at times. Beware, it says, this is a top 10 defense even while almost completely half-assing things, just on the virtue of Kevin Durant showing up to work. Durant has been improving as a defender for years, going back to his time with the Thunder. (If you ever go back and watch the 2016 series between OKC and San Antonio, watch Durant on defense — he was the best defensive big man in that series.) His style is also entirely his own: KD gives up relatively “great” shots — ones his opponents would be expected to turn into 52.3 eFG against an average defender, or about what Austin Rivers gives you as a defender. Not great. But because Durant is so long, so mobile and so smart positionally, the actual shots against him fall at a rate of just 45.2 eFG. This still happens within the overall Golden State system — Andre Iguodala and Steph Curry have similar opponent eFG numbers, albeit on fewer shots defended — but Durant’s role has him not only locking down his own man but also covering up for lapses by his teammates.Green may remain the more important defender for the Warriors — he’s the anchor, and what he does from his position is impossible to replicate. But Durant’s play so far has been the strongest on a Golden State defense that should end up being a top-3-type unit, at minimum. But despite the limitations, these metrics are a pretty good way to look at who’s having the best defensive season. A few players stand out on the list: Honorable mentionGiannis Antetokounmpo and Joel Embiid have both been outstanding through the first quarter of the season, but they don’t have quite the same track record as some of the players listed above. Just the same, neither seems to be a fluke. We’ll be a little more certain how permanent their new defensive excellence is by the All-Star break. The same goes for Kristaps Porzingis, who last season was in the tier just behind Gobert and Green for overall defensive value, but this year fell off early before making up some ground in the past few weeks.Some others seem more like early-season mirages — Josh Richardson, Gary Harris and Eric Gordon are all having excessively good years by the metrics but are playing way above the levels they had in previous seasons.Check out our latest NBA predictions. Anthony Davis, New Orleans PelicansFrom the time he was a rookie, Anthony Davis has had all the tools. He’d block shots and steal alley-oops and tug on his shorts and check the opposing point guard 30 feet from the basket. He came into the league as a destructive defensive force, but for reasons ranging from injury to scheme to sheer offensive burden, Davis’s aggregate effect on the defensive end hasn’t always lived up to the promise of those moments. This season, it has. Davis ranks seventh in the league in shots defended overall, and the defense gets 12.7 points per 100 possessions worse when he sits down. That’s nearly double his defensive on/off split from last season, which was already by far the best of his career.Partly this is because of his partnership with DeMarcus Cousins. Defensive intensity hasn’t always been Cousins’s strong point, and that’s still true today, but he has defended the most shots in the league, and done so while holding opponents to a middle-of-the-road 51.4 eFG on shots he defends. Not great, certainly, but it’s a respectable backstop for the defense.Because of Cousins’s role as a reliable constant, Davis is free to cover more ground without making risky moves to get back into plays. And because of that, his personal foul rate is at an all-time low (2.6 fouls per 100 possessions). This allows Davis to do what he does best — stick to his man, rotate to help in the paint without overextending, and blitz spot-up shooters on the perimeter faster than anyone else his size in the league. Here we’ve got total shots as well as the qSM (the difference a defender makes on each shot) plotted against each other to show who’s affecting how many shots to what degree. This isn’t perfect. For one, a great defender doesn’t simply challenge shots — he denies them from happening, forcing a team to reset its offense. But by the same token, great defenders also work their way back into plays, affecting shots by playing smart help defense and covering acres of ground. Another thing these numbers don’t reflect the is overall quality of the shot — there are a few defenders who don’t depress value of each individual shot by as much as others but who force opponents into low-quality shots in general (through hard work and smart positioning) so that the overall effect is the same. Marcus Smart, Jaylen Brown and Al Horford, Boston CelticsThe Celtic defense has continued to crush opponents. Boston is 18-4, with a league-leading defensive rating (100), the second-lowest effective field goal percentage allowed (48.6) and a roster full of live-bodied defenders who can switch practically any assignment. Even Kyrie Irving is chipping in on the effort. But while every Celtic is doing his part, some parts are more critical than others. It’s undeniably been the play from Smart, Brown and Horford that has carried the Celts so far.Smart does not possess the most graceful set of skills, but what he provides is unmistakable to anyone watching. He’s all over the floor, hounding ball handlers, bodying up on bigger players in the post and bumping cutting players he isn’t even guarding, like a middle linebacker jamming a slot receiver.Brown is a little different. While the Boston defense is built on interchangeable pieces switching and denying an offense space, it also needs players who can stick to their man across multiple screens, then square up and check their man as he plants and drives. Brown has stepped into that role and dominated so far. He can fight over screens (or just outright avoid them) to allow the defense to keep its shape, and he has the quicks to shut down first steps with the length to challenge pull-up attempts.Horford, meanwhile, does not have nearly the same reputation as the other two, but this season, he looks livelier than he has in the past when making switches, and he has defended the most shots of any Celtic. The newfound agility in space is especially important — because the defense will semi-regularly ask him to survive on an island against a wing, but also because his role at the center of the defense requires him to scuttle shooters who’ve just run over two or three screens trying to escape Smart or Brown.