The MLB offseason has been painfully slow when it comes to actual transactions, but at some point it's going to pick up and players will start to be signed. In the meantime, we're going to run through the positions and rank the available free-agent options. For one huge list, we have a free agent tracker with every position included. We've ranked catchers and first basemen so far. Now onto the middle infielders. 

Why the second basemen and shortstops together? Because it's just that thin a class. You'll see: 

Free agent middle infielders
Zack Cozart Free Agent SS
Free agency comes at the perfect time for Mr. Cozart. He's coming off a career year in which he hit .297/.385/.548 (141 OPS+) with 24 doubles and 24 home runs in just 438 at-bats. He walked (62 times) almost as much as he struck out (78), too. He's 32 and teams aren't as excited to pay for a career year, but he's a quality defensive shortstop with good offensive upside.
Neil Walker Free Agent 2B
Walker is also entering his age-32 season and he's a sub-par defender as a slightly less important position. That's why he's second and not first. What Walker does is hit. Every. Single. Year. Going by OPS+, Walker has been an above-average hitter every single season starting in 2010. He slashed .265/.362/.439 (111 OPS+) with 14 homers in 111 games last season for the Mets and Brewers. He'll be fully healthy to start next season as well. Last season at this time, he was recovering from back surgery.
Brandon Phillips Free Agent 2B
We're already into questionable starter territory. Phillips still has the name and can hit for average (.285 last year), but he doesn't walk (21 last year to give him a sub-par .319 OBP) and doesn't have good power anymore, relative to the rest of the power-surging league. He's coming off a 93 OPS+ and the once-outstanding defender is showing his age on the field. He rated out as below-average in Defensive Runs Saved in 2017 for the second straight season. He'll be 37 next year, too.
Chase Utley Free Agent 2B
Heading into his age-39 season, retirement is nigh for the former superstar. He's still pretty sure-handed at second, will take walks and has some gap power. He shouldn't be an everyday starter, but he can bring value off the bench and in spot starts.
Alcides Escobar Free Agent SS
Escobar is entering his age-31 season and has played in 162 games in three of the past four regular seasons. He's been a dreadful offensive player the past several years, though. Last season, he hit .250/.272/.357 (65 OPS+). He struck out 102 times but walked only 15. He was even terrible at stealing bases, succeeding just four times in 11 tries. His defensive metrics are trending in the wrong direction as well. If your team needs a starting shortstop in free agency, hope they pick someone else. Escobar needs to be a backup at this point.
Darwin Barney Free Agent 2B
Barney can play passable defense at second, third or short, so there's value in that. There's none in his bat. He hit .232/.275/.327 (57 OPS+) last year.
J.J. Hardy Free Agent SS
In 2013, Hardy won a Gold Glove and Silver Slugger, but he's in a big-time age decline. He only managed 73 games last year due to injury and hit .217/.255/.323 (55 OPS+). In a full season, he'll still play quality shortstop, but entering his age-35 season, his value can't be very high.
Erick Aybar Free Agent SS
Aybar is a slightly better offensive performer (.234/.300/.348) than the two listed above him, but he's range-deficient at shortstop and defense is more important for backup middle infielders.
Cliff Pennington Free Agent 2B
Pennington can handle second, third or short, but he's inferior to Barney. Though he was a bit better in 2017 than 2016, Pennington has hit .232/.287/.320 in the past two seasons combined.
Danny Espinosa Free Agent 2B
Last offseason, the Nationals traded Espinosa to the Angels for a couple of non-prospects. He was then released by two different teams during the regular season. In 295 plate appearances for three different teams, he hit .173 with a 43 OPS+. Let this be an illustration of why I decided to rank "middle infielders" instead of doing separate rankings of second basemen and shortstops.