When the NFL free-agency period begins in just a few days, it should have a defensive feel to it.

The offensive class doesn't offer a lot, but teams can get help on the defensive side of the ball, even though many of the top defenders are already off the market. 

Here are my top-20 NFL free agents:

Trey Flowers New England Patriots DE
He doesn't have great sack numbers, getting just 7.5 this season, but he is a force as a pass rusher. His versatility is what makes him so attractive. The Patriots would be wise to keep him.
Landon Collins New York Giants SS
He looked like a future Hall of Fame player in 2016, but his play has tailed off some and he ended last season on injured reserve with a partially torn labrum. Even so, he's 25 and he's a quality starter in a league where good safety play is imperative.
Trent Brown New England Patriots T
The Patriots traded with San Francisco to get him last spring and he became their starting left tackle. He played well – especially in the playoffs – and now this massive 25-year-old is about to get paid.
C.J. Mosley Baltimore Ravens ILB
His position isn't one of the value positions, but he's been a playmaker on a good defense who is good against the run and the pass. At 26, he's the right age.
Le'Veon Bell Pittsburgh Steelers RB
When Bell finally does play for a team, it will be for the first time since January of 2018, after sitting out last season in a contract dispute. Some will say that's a good thing for a back. I beg to differ. And he's a year older. That's not a good thing for a back. There is risk signing him to a mega-deal, but some team will do it.
Matt Paradis Denver Broncos C
He is coming off a season cut short by a broken leg. He has 57 starts in four seasons and has been a good interior player in Denver's offense.
Anthony Barr Minnesota Vikings OLB
He started slowly in 2018, missed some time with a hamstring injury, but bounced back to play well down the stretch. Given more chances to rush the passer, he flashed. That could help his value on the market.
Tyrann Mathieu Houston Texans FS
He had a nice first season with the Texans playing on a one-year deal. His ability to play all over the field could really help a defense. He's good against the run and the pass.
Earl Thomas Seattle Seahawks FS
Injuries are why he's down this list some, and he isn't a kid anymore – he'll turn 30 in May. But he can be a nice addition in the middle of the field for a defense in need of a playmaker. The question is how much does he have left.
Bryce Callahan Chicago Bears CB
He's one of the better nickel corners in the league. With teams playing so much nickel now, that means he's a quality starter. That will get him paid.
Ja'Wuan James Miami Dolphins T
This 26-year-old has 62 starts in five seasons, including 15 at right tackle in 2018. He is a solid right tackle who has a lot of good football in front of him.
Preston Smith Washington Redskins OLB
This 26-year-old is coming off his best season. He has 24.5 sacks the past four seasons, four in 2018. But he's much more disruptive than that number would indicate.
Za'Darius Smith Baltimore Ravens OLB
This is a player who should be getting more attention heading into the market. He's emerged as a quality pass rusher inside and outside – getting 8.5 sacks last season – and his versatility would have great value for a team.
Nick Foles Philadelphia Eagles QB
It looks like he's going to hit the market if the Eagles don't franchise tag him. If he does, he will be the top quarterback available. The price tag will be interesting.
Adrian Amos Chicago Bears SS
He's been a quality player on the back end for the Bears. He is good against the run and the pass, although he isn't as good as Eddie Jackson, who played next to him.
Tyrell Williams Los Angeles Chargers WR
He said he could be a No. 1 receiver for a team if given the chance. I think he's more of a high-end No.2 receiver, but his size and speed are enticing.
Rodger Saffold Los Angeles Rams G
He will be 31 this year, so age is a bit of a concern. But teams looking for a good run-blocking guard would be wise to take a look at Saffold.
Ndamukong Suh Los Angeles Rams NT
He wasn't great during the regular season for the Rams, but he picked it up in the playoffs to remind personnel people that he can still play at a high level. Motivation remains an issue.
Daryl Williams Carolina Panthers T
He was a force in 2017, but missed almost the entire season in 2018 with a knee injury. If that checks out, he could be a nice addition for a team looking for line help.
Ronald Darby Philadelphia Eagles CB
When he's on the field, he's a good corner. The problem is he's missed too much time the past two years. Even so, he's only 25.