Some NBA defenders are perimeter pests who snuff out isolation sets and pick-and-rolls against even the most talented offensive threats. Before going down for the season, Andre Roberson might've been the best example.

Others, like Rudy Gobert, are terrific on the interior at protecting the rim and dissuading drivers from entering their domain. Then there are the tremendous off-ball defenders such as Paul George, who use their athleticism, quick instincts and lanky arms to disrupt passing lanes.

But we're not talking about any stoppers who fall into those categories. 

These five players—and those who fall into the dishonorable mentions at each of the traditional positions—are best at serving as sieves. They usher opponents into the painted area, cede wide-open jumpers and fail to contest shots properly as the last line of defense.

They're liabilities. 

Defensive stats and metrics are notoriously faulty, but here we're blending together four different ones in an attempt to stay objective and focus on what's taken place during 2017-18 (looking only at players with at least 500 minutes logged):

• ESPN.com's defensive real plus/minus, which "takes into account teammates, opponents and additional factors" while looking at estimated on-off impact. 
• NBA Math's defensive points saved, which accounts for defensive volume and efficiency while also taking defensive rebounding into consideration.
• Field-goal percentage differential, which, while subject to plenty of noise, looks at how successful players are at forcing misses while factoring in the strength of opponents defended.
• On/off differential, which looks, in a vacuum, at a team's defensive strength with and without a player. 

To standardize these four metrics, which operate on drastically different scales, we found the z-scores in each category and summed them to find a player's total score. Those cumulative z-scores are all that matter for this countdown. 

Point Guard: Emmanuel Mudiay, New York Knicks (Minus-6.19)

DRPM Z-Score: minus-2.35

DPS Z-Score: minus-1.0

FG% Differential Z-Score: minus-2.07

On/Off Differential Z-Score: minus-0.77

Even though Emmanuel Mudiay entered the NBA as a raw prospect, he was supposed to have the size and frame necessary to hold his own defensively. A 6'5" floor general with a wingspan measuring in at 6'8.5" who boasted sizable amounts of athleticism should be able to get his body and arms into the way of opposing players. 

But three years later, Mudiay has emerged as a defensive detriment, and that's remained true with both the Denver Nuggets and New York Knicks.

The 22-year-old fares poorly in every one of our categories, but he's dragged down furthest by his putrid score in ESPN.com's DRPM, which places him ahead of exactly zero 1-guards among the 100 classified at the smallest position. In fact, the gap between Mudiay (minus-3.89) and No. 99 Isaiah Thomas (minus-3.11) is sized identically to the chasm between Thomas and No. 93 J.J. Barea. 

Mudiay is truly in a class of his own—which, in this case, is far from a positive. 

In the past, the floor general has used solid anticipatory skills to navigate pick-and-roll sets against opposing ball-handlers, and his size alone made him a solid isolation stopper. Off-ball situations always caused him to fall asleep, though. Troublingly, he's now drifting in the wrong direction while adversaries know better than to play straight-up ISO basketball against him. 

Last year, Mudiay ranked in the 18th and 68th percentiles for spot-up and PnR ball-handler defense, respectively. This season, he checks in at 16th and 50th. That's not the trend the Knicks are looking for, though they should feel pleased they didn't part with any first-round picks while acquiring his services.   

Class of the Position: Lonzo Ball, Los Angeles Lakers (5.67); Dejounte Murray, San Antonio Spurs (5.03); Ben Simmons, Philadelphia 76ers (6.18); Fred VanVleet, Toronto Raptors (3.41)

Dishonorable Mentions: Patty Mills, San Antonio Spurs (minus-4.64); D'Angelo Russell, Brooklyn Nets (minus-4.73); Dennis Schroder, Atlanta Hawks (minus-6.13); Dennis Smith Jr., Dallas Mavericks (minus-5.17)

Shooting Guard: Jamal Crawford, Minnesota Timberwolves (minus-9.15)

DRPM Z-Score: minus-3.01

DPS Z-Score: minus-2.27

FG% Differential Z-Score: minus-1.94

On/Off Differential Z-Score: minus-1.93

Breaking news: Playing defense as an NBA shooting guard is a difficult task, considering 2s are so frequently asked to loft plenty of attempts and are typically talented at tickling twine. The five worst overall scores in this analysis belonged to bigger backcourt members, though one managed to veer well clear of the pack. 

Not to spoil the dishonorable mentions, but rookie shooting guard Malik Monk emerged with the second-worst mark in the NBA for his inability to prevent points for the Charlotte Hornets during his first season out of Kentucky. But his score of minus-6.9 isn't even in the same ballpark as Jamal Crawford's minus-9.15. That gap of 2.25 points is about the equivalent of the canyon separating Monk from Patty Mills (minus-6.4), who is 18 spots ahead.

Of course, this shouldn't be too surprising. 

Crawford, with room to spare, is dead last in ESPN.com's DRPM regardless of position. NBA Math's DPS, which factors in volume, only has Lou Williams checking in with less value. Crawford allows defenders to experience far more success when he's guarding them—their field-goal percentage, on average, goes from 44.6 to 50.7—and the Minnesota Timberwolves' defensive rating shifts from 105.1 to 113.2 when he's on the floor. Nothing looks positive. 

Unfortunately, pinpointing one area of Crawford's defense that needs work is a tough task, since he's avoided exerting too much effort on the less glamorous end for years. Off-ball screens against him easily free up shooters. His feet seem to be cemented to the hardwood when he's defending in a one-on-one situation. He makes poor decisions in pick-and-roll scenarios. 

Crawford isn't just our shooting guard representative. He's the captain of this no-defense team. 

Class of the Position: Jimmy Butler, Minnesota Timberwolves (3.72); Victor Oladipo, Indiana Pacers (4.84); Andre Roberson, Oklahoma City Thunder (6.93), Marcus Smart, Boston Celtics (3.76)

Dishonorable Mentions: Marco Belinelli, Philadelphia 76ers (minus-6.49); Rodney Hood, Cleveland Cavaliers (minus-6.45); Malik Monk, Charlotte Hornets (minus-6.9); Lou Williams, Los Angeles Clippers (minus-6.45)

Small Forward: Evan Fournier, Orlando Magic (minus-4.21)

DRPM Z-Score: minus-0.97

DPS Z-Score: minus-1.84

FG% Differential Z-Score: minus-1.07

On/Off Differential Z-Score: minus-0.33

Part of defense is ending possessions with a secured rebound, and that's one of the areas in which Evan Fournier struggles.

His 3.1 defensive boards per 36 minutes are his highest mark since the 2013-14 campaign for the Denver Nuggets (4.0), but they still pale in comparison to what one might expect from a 6'7" wing playing major minutes. Among the NBA's 154 qualified players standing at least as tall as Fournier, only Terrance Ferguson (1.2), Tony Snell (2.3), Nick Young (2.5), Patrick McCaw (2.6), Reggie Bullock (2.7) and Darius Miller (2.7) are less effective in this statistic. 

But Fournier's struggles go well beyond rebounding. 

Early in the season, he received some credit for his improved effort on the stopping side, particularly as it related to quick, deflection-causing hands. The French swingman has logged 2.2 deflections per game throughout the 2017-18 campaign, but that number has dropped to 1.9 since the beginning of December. In 2018 alone, it's fallen to 1.7.

Fournier has begun looking more tired—perhaps a byproduct of shouldering an important offensive role for the struggling Magic, or potentially a lingering effect from an ankle injury that slowed him in early December. Regardless, it's affected his pick-and-roll coverage against opposing ball-handlers (26th percentile), and he's had trouble recovering to spot-up shooters in a timely fashion (49th percentile). 

This 25-year-old might not look the part of a defensive sieve, largely because you can watch him in quite a few areas and see someone displaying a bit of effort while not getting fully torched. But the consistently below-average play across the board tends to add up, especially at a position that features so many legitimate stoppers. 

Class of the Position: Kyle Anderson, San Antonio Spurs (4.88); Jaylen Brown, Boston Celtics (4.28); Andre Iguodala, Golden State Warriors (4.72); Royce O'Neale, Utah Jazz (4.32)

Dishonorable Mentions: Luke Babbitt, Miami Heat (minus-3.41); Jae Crowder, Utah Jazz (minus-4.16); Mario Hezonja, Orlando Magic (minus-3.14); Wesley Matthews, Dallas Mavericks (minus-3.59)

Power Forward: Zach Randolph, Sacramento Kings (minus-5.52)

DRPM Z-Score: minus-0.82

DPS Z-Score: minus-0.89

FG% Differential Z-Score: minus-2.36

On/Off Differential Z-Score: minus-1.45

Players whose vertical jumps are best measured in millimeters don't tend to fare particularly well on the defensive end. The same is true of lumbering big men without much lateral mobility. Ditto for veteran power forwards embarking upon their age-36 seasons while surrounded by youthful perimeter talents not yet skilled at preventing dribble penetration. 

Unfortunately for Zach Randolph, he fits into each of those categories. 

During his prime years, Randolph could leverage his physicality and the strength of his base into high-quality defense. But that's no longer the case, since he's become so immobile that even slow-footed offensive threats can treat him like a traffic cone. He's now hemorrhaging an even point per possession against post-ups, which leaves him reeling in the 23rd percentile. 

On/off stats can often be noisy, subject to variation that stems more from lineup combinations than individual efficacy. But they're also telling in this situation, as the Sacramento Kings desperately need a capable rim-protecting presence behind their burgeoning backcourt members. Randolph is often replaced by younger, springier bigs when he moves to the pine, which is a major reason his squad allows 6.2 fewer points per 100 possessions without him patrolling the paint. 

This isn't a multicollinearity effect, either. Among rotation members still on the roster, De'Aaron Fox (2.8 worse), Bogdan Bogdanovic (1.7 worse) and Willie Cauley-Stein (0.3) are the only other Kings with negative splits. 

Randolph, for all the good he can still add in the scoring department and as a passionate leader, has become a matador. 

Class of the Position: Giannis Antetokounmpo, Milwaukee Bucks (6.94); Anthony Davis, New Orleans Pelicans (6.88); Draymond Green, Golden State Warriors (6.42); Luc Mbah a Moute, Houston Rockets (5.49)

Dishonorable Mentions: Carmelo Anthony, Oklahoma City Thunder (minus-3.81); Harrison Barnes, Dallas Mavericks (minus-3.86); Frank Kaminsky, Charlotte Hornets (minus-4.8); Lauri Markkanen, Chicago Bulls (minus-3.4)

Center: Tristan Thompson, Cleveland Cavaliers (minus-3.94)

DRPM Z-Score: minus-0.78

DPS Z-Score: minus-0.25

FG% Differential Z-Score: minus-1.78

On/Off Differential Z-Score: minus-1.13

First, let's address the elephant in the room. 

Yes, Tristan Thompson qualifies as a center this year. Basketball-Reference.com shows that 76 percent of his minutes have come at the 5 (the second consecutive season he's spent the majority of his time at center), while Cleaning the Glass indicates that he's been exclusively there for two straight go-rounds. Forget about his spot in the lineup as a younger player. 

Now that said, Thompson's spot has often come next to head coach Tyronn Lue in 2017-18; he's played just 21.4 minutes per game because the Cleveland Cavaliers can't withstand yet another point-preventing liability in a lineup often filled with such players. Bad as the defense has been throughout the campaign, it's still been 4.2 points per 100 possessions worse when he's logging minutes.  

In previous seasons, Thompson was far from a negative defensively.

His mobility on the perimeter gave him the ability to switch screens against small-ball lineups while also covering roll men and serving as a last line of defense on the interior. He fit perfectly into matchups against three-happy teams such as the Golden State Warriors—a role he might once again resume in the playoffs if he improves and the right set of assignments beckons.

But after finishing 2016-17 in the 62nd percentile as a roll-man defender, he's now down to the 37th percentile. He's letting opponents shoot 70.4 percent at the rim—a stark contrast to 54.5 percent last year and 57.5 percent back in 2015-16. The 27-year-old hasn't gotten slower or developed lazy habits so much as he's struggled to mesh with the changing personnel populating Northeast Ohio. 

Class of the Position: Joel Embiid, Philadelphia 76ers (7.54); Rudy Gobert, Utah Jazz (7.24); Al Horford, Boston Celtics (6.11); David West, Golden State Warriors (7.98)

Dishonorable Mentions: Cristiano Felicio, Chicago Bulls (minus-3.62); Kevin Love, Cleveland Cavaliers (minus-3.15); Enes Kanter, New York Knicks (minus-3.18); Greg Monroe, Boston Celtics (minus-2.99)

Adam Fromal covers the NBA for Bleacher Report. Follow him on Twitter: @fromal09.

Unless otherwise indicated, all stats from Basketball Reference, NBA.com, NBA Math or ESPN.com and are current heading into games on March 12.