The hottest commodity at MLB's Winter Meetings was relief pitchers. While marquee hitters like J.D. Martinez and Eric Hosmer remain unsigned, as do top starting pitchers like Yu Darvish and Jacob Arrieta, relievers were signing left and right.
And not just any old relievers. With teams growing ever more analytically inclined, the bullpen arms getting snapped up had some of the best advanced metrics of anyone available on the open market.
Let's use Statcast™ to look at the slate of signees -- specifically, expected wOBA. Weighted on-base average is an overall measure of performance, weighing different outcomes based on their value (e.g. a home run is worth more than a single). Statcast™ can take a pitcher's walks, strikeouts and quality of contact allowed -- using the combinations of exit velocity and launch angle on every batted ball against him -- and generate an expected wOBA, or xwOBA for short.
At week's end, eight of the top 10 free-agent relief pitchers by 2017 xwOBA had already been signed. One of the two remaining, Matt Albers, is expected to sign a two-year deal this week, according to ESPN's Jerry Crasnick. The other, Greg Holland, could easily sign soon as well.

Here's a rundown, in order of their xWOBA from last season, of the top relievers who have inked new deals, and the best still available. You can see the overall relief pitcher xwOBA leaderboard here.
Pat Neshek (RHP): Phillies
2017 xwOBA: .216
Overall RP xwOBA rank: 2 (of 191 relievers to pitch at least 150 AB in 2017)
Neshek returns to the Phillies after a superb 2017 season split between them and the Rockies. The 37-year-old had a 1.59 ERA, 69 strikeouts and just six walks in 62 1/3 innings, and hitters didn't square up many of the offerings that emerged from his unorthodox sidearm delivery -- Neshek's 26.9 percent hard-hit rate was 13th-lowest of 114 relievers with 150 or more batted balls against them. (Statcast™ defines a hard-hit ball as having an exit velocity of 95-plus mph.)

Tommy Hunter (RHP): Phillies
2017 xwOBA: .234
Overall RP xwOBA rank: 7
The Phillies actually nailed down the top two relief options available by xwOBA. Hunter, who had a 2.61 ERA and 64 strikeouts in 58 2/3 innings for the Rays in 2017, slotted in at seventh among relievers with a .234 xwOBA, just one point behind Andrew Miller and Roberto Osuna and two behind Craig Kimbrel. The 31-year-old mixed his pitches very evenly and located them well -- including, notably, up in the zone with his high-spin four-seamer, which averaged a 2,463 rpm spin rate this past season. High-spin fastballs tend to get swings-and-misses in those spots.

Mike Minor (LHP): Rangers
2017 xwOBA: .238
Overall RP xwOBA rank: 8
The Rangers may end up using Minor as a starter, but that doesn't change the fact that he was one of baseball's most effective relievers for the Royals this year. In 65 appearances for Kansas City, Minor had a 2.55 ERA and struck out 88 batters in 77 2/3 innings. Not only is the 29-year-old a hard-throwing lefty -- Minor's fastball averaged 94.6 mph in 2017 -- but that four-seamer also has one of the highest spin rates in baseball. It averaged 2,604 rpm this season, which was the fourth-highest of any pitcher who threw even 100 fastballs in 2017.
Brandon Morrow (RHP): Cubs
2017 xwOBA: .240
Overall RP xwOBA rank: T-9
Morrow has remade himself as a lights-out reliever in his early 30s, and he added more than 3 mph to his average fastball velocity from 2016 to '17. It's normal for a pitcher to throw harder out of the bullpen than as a starter, and that helped the 33-year-old post a 2.06 ERA with 50 strikeouts in 43 2/3 innings for the Dodgers last season, and now he heads to the NL Central champs.

Joe Smith (RHP): Astros
2017 xwOBA: .241
Overall RP xwOBA rank: T-11
Smith joins the World Series champion Astros after a nice bounceback year in 2017. The sidearmer struck out 71 batters in 54 innings, producing a career-best 11.8 strikeouts per nine innings. Smith cut 118 points off his xwOBA from 2016 to '17, going from the bottom 10 percent to the top 5 percent of relievers in that category. The 33-year-old's improvement was the second-largest of any reliever to pitch at least 100 at-bats in both 2016 and '17.

Anthony Swarzak (RHP): Mets
2017 xwOBA: .245
Overall RP xwOBA rank: 16
After struggling with the Yankees in 2016, Swarzak had the best season of his career this year. The 32-year-old set career highs in basically every traditional category: 70 appearances, a 2.33 ERA, 91 strikeouts and 10.6 strikeouts per nine innings. His slider was his most-used pitch -- he threw it more than half the time -- but his fastball was his best pitch. Swarzak's four-seam averaged 94.8 mph, 2 mph harder than in 2016, and opponents hit just .158 and slugged .252 against it. 

Matt Albers (RHP): Unsigned
2017 xwOBA: .246
Overall RP xwOBA rank: T-17
Albers is the top unsigned free-agent reliever by xwOBA, and he could be off the board in the coming week according to one prominent report. The 34-year-old had a great year for the Nationals last season, with a 1.62 ERA and 63 strikeouts in 61 innings over 63 appearances. 
Bryan Shaw (RHP): Rockies
2017 xwOBA: .260
Overall RP xwOBA rank: T-33
Like Smith, Shaw was an important part of the Tribe's relief corps in their run to the AL Central crown. He led the AL in appearances for a second straight season and the third time in the last four, pitching in an MLB-high 79 games and a career-high 76 2/3 innings. Shaw's ground-ball rate was 59 percent, which could make the 30-year-old a good match for Coors Field, where you want to keep the ball out of the air.

Yusmeiro Petit (RHP): A's
2017 xwOBA: .265
Overall RP xwOBA rank: 40
Petit had one of his best Major League seasons for the Angels, and his 2.76 ERA and 101 strikeouts in 91 1/3 innings almost exclusively as a reliever helped net the 33-year-old a multiyear deal with the A's. Opponents hit under .200 against all of Petit's secondary pitches -- .184 in 87 at-bats against his curveball, .192 in 73 at-bats against his changeup and .128 in 39 at-bats against his cutter.
Greg Holland (RHP): Unsigned
2017 xwOBA: .270
Overall RP xwOBA rank: T-43
Holland could still end up re-signing with the Rockies after earning NL Comeback Player of the Year honors as their closer in 2017, but the 32-year-old remains unsigned for the time being. Holland threw his slider at a career-high 49.8 percent rate, and it was both his heaviest-used and most effective pitch. Opponents hit just .137 and slugged .226 against Holland's slider, with 102 swings-and-misses. Holland got whiffs on 47.7 percent of the swings against his slider, eighth-highest of the 80 pitchers who threw 200-plus sliders in 2017.

Who else is unsigned, and how did they rank?
There are some other notable names out there on the free-agent relief market who didn't rank in the top 10 in xwOBA. Here's a look at a few:
• Wade Davis (RHP)
2017 xwOBA: .289
Overall RP xwOBA rank: T-86
Widely considered one of baseball's top closers, Davis was the biggest name available when the offseason began. He still is, but his xwOBA was surprisingly high compared to the last two years -- although it was easily better than MLB average for a reliever, which was .304 in 2017.

• Addison Reed (RHP)
2017 xwOBA: .300
Overall RP xwOBA rank: T-111
Reed's overall 2017 numbers looked pretty good -- the 28-year-old saved 19 games for the Mets before being traded to the Red Sox, and he finished the year with a 2.84 ERA and 76 strikeouts in 76 innings. Reed's calling card is that he pounds the strike zone -- of the 280 Major League pitchers who threw 1,000 pitches in 2017, his 55.9 percent in-zone rate was the second-highest. He issued just 15 walks on the season, although his walk rate did bump up a little after he arrived in Boston.
• Tony Watson (LHP)
2017 xwOBA: .277
Overall RP xwOBA rank: T-60
The left-handed reliever market was especially thin entering the offseason, and Watson is maybe the strongest option left with Minor and Jake McGee (Rockies, .278 xwOBA) now both signed. The 32-year-old had a 3.38 ERA in 71 appearances between the Pirates and Dodgers, and he turned in scoreless outings in nine of his 11 postseason games.