The key with defenses is finding defenses that will face a lot of passing plays. Fantasy point-scoring events like interceptions and sacks occur exclusively on passing plays, and fumbles actually occur more on passing plays than running plays. Defensive touchdowns are difficult to predict, but they're also more likely to occur on passing plays with 53 pick sixes last year compared to only 33 scoop 'n scores.
One of the best ways to identify defenses that will face a lot of passing plays is identifying Vegas favorites, preferably home favorites. And obviously the lower the opponent's implied point total, the better the matchup for the defense.
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The chalk this week looks like it's going to be Miami facing off with Jared Goff, but let's get back to them in a moment. The slightly less chalky play but the preferred chalk play is Kansas City at home against Tampa Bay. Tampa Bay has the lowest implied total in Vegas at 18.5, and they've given up some big fantasy point totals to good defenses when they've faced them this season. The Arizona and Denver D/STs each topped 15 fantasy points when they played Tampa. Kansas City isn't quite on the level of those defenses, but they are a good one as they rank ninth in Football Outsiders' Defense Efficiency Ratings (DVOA). Kansas City as a 7.5-point home favorite against a team with the lowest implied total screams cash game play.
As mentioned, Miami is going to be super chalky this week as they get the privilege of facing Jared Goff in his first start. Miami ranks fourth in pass DVOA and seventh overall, so they're a very good unit that's a bear of a matchup for Goff to walk into. That said, this game is a pick 'em and Miami is on the road, so the Vegas variables don't necessarily set up in Miami's favor. You have to imagine LA is going to do all they can to grind this game out, which means the fantasy-point scoring opportunities might be limited for Miami's D. The down side is limited to be sure, so they're still a good cash game play. But the upside might not totally be there, and they're going to be too chalky to use in GPPs anyway.
Sneaky Options
This category is reserved for good defenses that may be overlooked in a given week for whatever reason, providing you with the opportunity to roster a really good unit with a lower ownership rate.
Two teams fit that bill this weekend, Seattle and Minnesota. Both are home favorites (though Minny is only a slight favorite), and they're hosting teams with an implied total of 20 or less. The teams rank fifth and sixth respectively in DVOA, so these are good units in a good spot. The Seahawks are the bigger favorite as a 6.5-point favorite at home against Philly, but Minny has the better matchup against Arizona who has allowed the seventh most fantasy points per game to opposing defenses. Neither team figures to be too heavily owned, so they make for interesting GPP plays. Minnesota is priced very reasonably on both major sites while Seattle is the most expensive D on each site, though that should help keep their ownership rates down.
Bargain Option
If you're looking to go as cheap as you can, Dallas may be your best bet on either site. The Cowboys aren't a great defense as they rank 19th in DVOA, but they have an unimposing matchup against Baltimore. For one thing, Baltimore has the second lowest implied total on Sunday at only 19 points, and they rank 29th in FO's Offense Efficiency Ratings. Baltimore ranks 29th in pass DVOA as well, which makes them a good matchup for Dallas who struggles more against the pass than they do against the run. Dallas is a seven-point favorite at home in this one, so the matchup sets up well for them to see plenty of fantasy point scoring opportunities.