Throughout the preseason, we’ll take a look at the best – and worst – the NFL has to offer, unit by unit. Look for a new set of rankings every few days.

Many would agree that the hardest position to learn and excel at in football is quarterback. After all, the QB has to be the voice of the huddle, the audible caller, and he has to know what every offensive player is going to do on a given play.

Taking nothing away from the role of a quarterback, however, being a defensive back in the NFL is as mentally challenging as it is physically.

Unlike a wide receiver that knows where his route will take him, a cornerback’s first step is usually backward and the art form of the position relies on the player being good at reacting. A cornerback can stop and start his movement again and again a multitude of times before the play is even over with.

Much like a cornerback, a safety has to be able to tackle like a linebacker but cover the pass like a corner. Being a safety requires aggression, versatility and flexibility, and although they can see the whole field in front of them, safeties can get just as turned around during a given play as a cornerback.

Criteria for the secondary:

The starting four in a team’s secondary unit will account for 70% of the overall ranking placement.

The final 30% will go to the teams nickel back, since nickel coverage is used extensively during a game, as well as the overall depth a team carries in its secondary.

1. Denver Broncos
Champ Bailey, Darrent Williams, John Lynch, Nick Ferguson, Domonique Foxworth
The Broncos have a deadly combination: talent and youth. Besides having one of the best cornerbacks in the NFL in Champ Bailey, they have two second-year players that had a knack for making plays as rookies. Darrent Williams probably has a leg up on Domonique Foxworth in the battle for the starting corner spot, but Foxworth provides Denver with excellent depth and could start in a second if Williams were to go down. With 14 years of experience, SS John Lynch, who made the Pro Bowl in 2005, still plays at a high level and hits as hard as he did in Tampa. FS Nick Ferguson had a career-best five interceptions last year. As for Bailey, he excels at all phases of the cornerback position by being able to cover an opposing team’s No.1 wide out one-on-one and also help out in the run. Bailey made his sixth Pro Bowl appearance last year despite being hampered a bit by injuries.

2. Chicago Bears
Charles Tillman, Nathan Vasher, Mike Brown, Chris Harris, Ricky Manning Jr.
Charles Tillman might be the most acclaimed cornerback on this team, but Nathan Vasher is a better playmaker. That statement has less to do with Tillman’s abilities, which are great, and more to do with Vasher’s aggressive style of play and superior on-the-ball skills. Tillman is excellent against the run, but sometimes gets torched by a No. 1 receiver (remember what Carolina’s Steve Smith did to him last season?). When healthy, SS Mike Brown is an explosive hitter that offsets his lack of elite speed with supreme intelligence. FS Chris Harris excels at playing centerfield in the Bears’ defense as both a ball hawk and a solid run supporter. Chicago signed former Panther Ricky Manning Jr. in the offseason to be the nickel back. Manning is quick and can take on a team’s slot receiver. Rookie Danieal Manning is built like a safety, but has cornerback skills.

3. Cincinnati Bengals
Deltha O’Neal, Tory James, Dexter Jackson, Madieu Williams, Keiwan Ratliff
The secondary that produced the most interceptions in the NFL last season returns at full strength for ’06. Deltha O’Neal finished tied with Ty Law for the league lead in interceptions last year with 10 while fellow corner Tory James added five. Adding SS Dexter Jackson to the mix this year will shore up the run defense and the return of Madieu Williams from a shoulder injury will greatly improve the Bengals’ safety unit this season. Keiwan Ratliff will get a chance to challenge James for more playing time, but in the meantime he is a solid nickel corner.

4. Dallas Cowboys
Terence Newman, Anthony Henry, Roy Williams, Keith Davis, Aaron Glenn
Although the Cowboys didn’t come away with many interceptions, the secondary unit gave up the lowest completion percentage of any team last season. After a sophomore slump, Terence Newman bound back in ’05 to have a Pro Bowl-caliber season and should have continued success in 2006. SS Roy Williams might be the league’s most feared hitter over the middle and in run support, but some critics feel that he has to get better in pass protection. Not many people were aware that CB Anthony Henry was Dallas’ best defender last year before he got hurt and Aaron Glenn was very productive at the nickel last season. Keith Davis is a bit of a mystery right now after being shot twice this offseason. If he does return to full health, Davis needs to work on his pass protection.

5. Philadelphia Eagles
Lito Sheppard, Sheldon Brown, Brian Dawkins, Michael Lewis, Roderick Hood
What happened to this fine group in ’05? Three out of the four starters made the Pro Bowl for Philly during its Super Bowl run, but the same unit was shredded by injuries and overall lackluster play in ’06. Brian Dawkins is fierce, smart and has just as much playmaking ability as any safety in the league. Lito Sheppard took a step back last year because of a high ankle sprain and a bad knee, but the injury gave Roderick Hood an opportunity to show what he can do in a starting role as he established himself as possibly the best nickel corner in the league. Sheldon Brown will start at the other corner spot opposite Sheppard and he is by far one of the game’s surest tackling corners. Michael Lewis had a sub par ’05 season, but so did the rest of this secondary.

6. Washington Redskins
Shawn Springs, Carlos Rogers, Sean Taylor, Adam Archuleta, Kenny Wright
Caution: this rating is based on the assumption that safety Sean Taylor will spend more time on the field more than in prison in ’06. Taylor is a terrific athlete, a solid hitter, but just can’t seem to stay out of trouble. Shawn Springs is 30, but still turned in a Pro Bowl-type performance last year and is a strong model for fellow corner Carlos Rogers. Rogers was the ninth pick in the ’05 draft and is looking to improve on an average but promising rookie season. Adam Archuleta comes over from the Rams and although he is fantastic against the run, his free agent price tag might have been a tad high considering his coverage skills are sketchy at best. Kenny Wright did a nice job as a starter in Jacksonville last season and should be a good third corner for the ‘Skins this year.

7. Pittsburgh Steelers
Ike Taylor, Deshea Townsend, Troy Polamalu, Ryan Clark, Bryant McFadden
Ike Taylor is a very underrated cornerback who led the NFL with 24 passes defensed in 2005. Deshea Townsend will compete with Bryant McFadden for the starting spot on the opposite side of Taylor, but whoever doesn’t start will provide excellent depth. McFadden showed outstanding play in nickel and dime situations last year, even going one-on-one with Colts stud wide out Reggie Wayne at the end of the Steelers-Indy playoff game. Troy Polamalu is a non-stop playmaker who always finds himself in the right position to attack the ball carrier or knock down a pass. Here’s the “but” in Pittsburgh’s secondary this year: they’ll miss Chris Hope more than they think. Ryan Clark is good against the run, but Hope opened up doors for Polamalu because of the way he covered centerfield for the Steelers during their Super Bowl run.

8. Tampa Bay Buccaneers
Ronde Barber, Brian Kelly, Jermaine Phillips, Will Allen, Juran Bolden
Ronde Barber and Brain Kelly still form one of the best cornerback duos in the NFL. Barber is a carbon copy of Champ Bailey – a guy that can play the run as well as the pass and is a savvy veteran. Kelly gets overshadowed a lot by Barber’s play, but he is outstanding against the pass considering he gets a lot of work with Barber on the other side. This group takes a small slide in its safety duo. Jermaine Phillips is usually hampered by injuries but can deliver a tough hit when healthy, and Tampa Bay feels that Will Allen is finally ready to be a starter. Juran Bolden isn’t spectacular, but as a nickel corner, he’s reliable and has tons of experience.

9. Carolina Panthers
Ken Lucas, Chris Gamble, Shaun Williams, Mike Minter, Reggie Howard
Carolina had one of the best pass defenses in the league last season, largely because of the strong play of Ken Lucas and Chris Gamble. Both Lucas and Gamble are stout against the pass with Lucas being one of the best free-agent acquisitions in the entire league last year. Gamble will do just that, gamble on routes, but he has a knack for the big play. The reason why the Panthers are this low, however, is the same reason Tampa Bay is ranked where they are. Their safety play is questionable and so is the nickel spot. Mike Minter is a born leader and is excellent in the locker room and while defending the run. But both he and Shaun Williams are a bit unsteady in passing situations. Colin Branch will challenge Williams, who is returning from a serious injury last season. Obviously, Carolina didn’t think Ricky Manning Jr. was the best option at nickel back last season since they let him escape to Chicago, but Reggie Howard might not be a significant upgrade.

10. Baltimore Ravens
Ed Reed, Samari Rolle, Chris McAlister, Dawan Landry, David Pittman
If the Ravens had a real option at safety next to Ed Reed, they would easily be ranked in the top five. The loss of Will Demps will hurt the Ravens as much as the Steelers losing Chris Hope. Dawan Landry or whatever other player Baltimore plugs in at the strong safety spot will more than likely deliver a significant drop in production from Demps’ 2005 campaign. Another issue is that Reed, Samari Rolle and Chris McAlister all had a drop off coming from the 2004 season. Reed was the NFL Defensive Player of the Year in ‘04, but was less of a factor in ’05 while McAlister had just one interception and missed the Pro Bowl for the first time since 2002. Rolle is an elite cover corner but struggles against the run, and rookie David Pittman is a bit of a question mark as Baltimore’s nickel. The Ravens may go with a 4-4-3 defense in 2006, allowing the team to get its best 11 players on the field at all times. If the trio of Reed, Rolle and McAlister can get back on track, this might not be a bad move by Baltimore’s staff.

The Bottom Five:

28. Oakland Raiders
Nnamdi Asomugha, Fabian Washington, Stuart Schweigert, Michael Huff, Duane Starks
What in the name of all that is holy happened to the Oakland Raiders’ cornerback situation? Charles Woodson and Philip Buchanon were supposed to be a menacing first-round pick duo that the Raiders could rely on for seasons upon seasons to come. Fabian Washington is a fine player who should continue to get better, but there’s nothing special about safe-as-they-come Nnamdi Asomugha. Asomugha doesn’t really allow anyone to beat him deep, which is fine, but he isn’t a ball hawk either, as evidenced by his zero (ZERO) interceptions in 43 career games. First-round pick Michael Huff has a chance to be really good, but I doubt he’ll make a considerable splash this season. Stuart Schweigert can be a leader but must be more consistent in coverage, and what in the world did Oakland see in New England’s secondary last season that made them go out and get Duane Starks and Tyrone Poole?

29. San Francisco 49ers
Shawntae Spencer, Walt Harris, Mike Adams, Tony Parrish, Sammy Davis
Walt Harris can still be effective, but he’s slowing down at this point in his career. Shawntae Spencer can be a pleasant surprise at times, but struggles against elite receivers. Tony Parrish is solid and dependable, but how he recovers from his first career serious injury (broken leg) will determine how this group produces this season. Mike Adams was an undrafted player in 2004 and worked his way into the starting lineup last season. His job isn’t secure, however, as head coach Mike Nolan loves the newly acquired Chad Williams.

30. Houston Texans
Dunta Robinson, Lewis Sanders, Demarcus Faggins, C.C. Brown, Glenn Earl
This unit has one stud in Dunta Robinson and is very below average everywhere else. The saving grace for Houston, however, is that C.C. Brown and Glenn Earl have shown promise at the safety position, but nobody knows what to expect heading into ’06. Lewis Sanders and Demarcus Faggins will battle for the CB spot opposite Robinson while Phillip Buchanon is set to waste yet another opportunity to be a starter in the league.

31. San Diego Chargers
Quentin Jammer, Drayton Florence, Terrence Kiel, Marlon McCree, Antonio Cromartie
There is a strong argument that the Chargers should be where the Rams are ranked, but this group shows a little more upside than St. Louis does in my opinion. No cornerback gave up as many touchdowns last year as Quentin Jammer, which doesn’t vibe with his number-two overall selection in the 2004 draft. Rookie Antonio Cromartie will push Drayton Florence, but whichever player starts there will either be mediocre or average at best. Marlon McCree was a nice addition to the safety unit this offseason from Carolina, but he will have less help around him this year. The other safety spot is unsettled, which is a problem in and of itself. Terrence Kiel and Bhawoh Jue will compete for the job.

32. St. Louis Rams
Jerametrius Butler, Tye Hill, Oshiomogho Atogwe, Corey Chavous, Fakhir Brown
The players in this unit are either inexperienced, past their prime or just not very good. The secondary suffered greatly last year when Jerametrius Butler when down for the season with an ACL tear. He should be ready by the start of the season, but confidence and mobility will be a concern for Butler early on. Rookie corner Tye Hill can cover, but rookie DBs tend to struggle their first year and he’ll have to fight with average Fakhir Brown to even earn the starting job. The safeties are a mess. Corey Chavous is a great leader and he’s smart, but his skills have diminished, and Oshiomogho Atogwe showed signs of ability at the end of last season but has been hampered by injuries himself.