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Just how hard is it to predict which fantasy defenses will score the most points? Well, let's look at 2008 just to get an idea.

First, here were the 10 most popular defenses in fantasyland to begin the season, according to Average Draft Position (ADP) in ESPN standard leagues, and how they fared in ESPN standard scoring:

Top 10 fantasy defenses: ADP (2008)
Team	ADP	Performed
San Diego Chargers	1	19
Chicago Bears	2	6
Seattle Seahawks	3	21
Minnesota Vikings	4	8
New York Giants	5	10
Dallas Cowboys	6	14
New England Patriots	7	20
Indianapolis Colts	8	9
Green Bay Packers	9	11
San Francisco 49ers	10	25

And here's how the actual top fantasy defenses in 2008 were selected, via ADP in ESPN standard leagues, before the season began:

Actual Top 10 fantasy defenses (2008)
Team	ADP	Performed
Baltimore Ravens	14	1
Pittsburgh Steelers	11	2
Philadelphia Eagles	13	3
Tennessee Titans	15	4
Tampa Bay Buccaneers	16	5
Chicago Bears	2	6
New York Jets	ND*	7
Minnesota Vikings	4	8
Indianapolis Colts	8	9
New York Giants	5	10

* Not drafted in the average ESPN standard league.

People, that is some seriously shoddy drafting. And here's the thing: It happens nearly every year. In '07, the fantasy world had the Ravens as the best fantasy defense in football, and Baltimore finished 23rd. That same year, the Dolphins were supposed to have the seventh-best fantasy defense, and Miami finished dead last. Now, that's not to say fantasy defenses are entirely random all year long the way, for example, fantasy kickers often are. Once a defense has established itself as viable for fantasy in a season's first month, it usually stays that way. And the good news is that in a 10- or even 12-team league, there are almost always plenty of likely suspects on the waiver wire. But the point here is don't take a defense early in your draft. Let's look at the key storylines for fantasy defenses heading into '09.

The Elite

The Super Bowl champion Steelers allowed the fewest points and yards in the NFL last season, paced by defensive player of the year James Harrison and fellow linebacker LaMarr Woodley. There's definitely some age on this unit: Five of their six best defensive linemen are over the age of 30. But the way this team gets to the quarterback (51 sacks in 2008 were the second most in the league), the Steelers are a pretty safe bet. … My personal guess is that the Giants will have the best defense in football this year. They have so much depth that they're almost injury-proof. Osi Umenyiora returns from a knee injury, while defensive tackles Chris Canty and Rocky Bernard and linebacker Michael Boley jump into a ridiculously deep rotation in the front seven. While everyone else will be wearing down at the end of games and the end of the season, New York will just be heating up. … The Titans surely will miss Albert Haynesworth, but they were smart not to pay the kind of cap-busting money Washington did to get him. They have a ready-made replacement in the middle in Jason Jones, and the other 10 starters are back from a defense that created the most turnovers in the NFL last year. If Tennessee is going to run into problems, it'll be because of injuries; this unit isn't deep. … Like the Titans, the Ravens lost their defensive coordinator this winter (ex-Titans helmsman Jim Schwartz is now the head coach in Detroit; ex-Ravens coordinator Rex Ryan is now the head man for the Jets). Unlike the Titans, the Ravens were raided by their old coach. Bart Scott, Marques Douglas and Jim Leonhard all left for the Big Apple, and Baltimore's starting cornerbacks, Chris McAlister and Samari Rolle, are also gone. There's more downside drafting this defense than you might expect, though if Ray Lewis and Ed Reed are healthy, they should stay very productive. … Blitzing defenses are good fantasy defenses. That bromide is proven true nearly every season by the Eagles, who bring heat to an almost unprecedented degree every week. The front seven here is great. But blitzers had better get to the passer, and quick, because other than Asante Samuel, the secondary is all new. Ellis Hobbs (like Samuels a former Patriot) is here now; Sean Jones will replace Brian Dawkins; and Quintin Mikell gets a promotion at free safety.

Not Sexy, But They Get The Job Done

What keeps the Vikings' defense from elite status is the uncertainty surrounding the team's twin defensive tackle towers, Kevin Williams and Pat Williams. Those guys are still battling a league suspension for a banned substance that was never upheld in '08; as of now, the NFL plans on having both of the Williamses miss the first four games in '09. If they do, one of the league's best run defenses suddenly gets softer. There are also questions about the secondary in Minnesota: Are Antoine Winfield and Cedric Griffin ready to live up to their hype at corner? However, you should get a bevy of sacks from Jared Allen, and eventually the Williamses will return to bottle up the run. … As I mentioned above, Rex Ryan left Baltimore to take over the head job with the Jets, and he has all sorts of talent on that defense. Newly acquired Bart Scott is a beast at middle linebacker; new corner Lito Sheppard hopes to fill the void opposite stud cover man Darrelle Revis; David Harris is also great in the middle; and nose tackle Kris Jenkins should be recovered from a bad back that sapped him late in '08. If there are issues here, they're with the ends and outside linebackers. But if big-money-man Calvin Pace and veterans Shaun Ellis and Bryan Thomas bring it this year, watch out.

Joey Porter was basically horrible his first year in Miami, but in his second year he put the Dolphins back on the map, finishing second in the NFL in sacks with 17.5. He and Channing Crowder make a nice linebacker duo. Plus, I don't know what got into Will Allen last year, but suddenly he could cover anyone, and if that continues, it'll be a bonus. The secondary has the most questions, especially free-agent acquisition Eric Green. But if their highly drafted rookie corners (Vontae Davis and Sean Smith) can play, Miami should be all right. … Julius Peppers wants out of Carolina, but he didn't get dealt before the April draft, which makes it seem like he'll probably wind up playing another season for the Panthers, under the franchise tag. While Peppers' teammates didn't see him at any summer OTAs, they're expecting him at training camp. And if he shows and is motivated, the Panthers will be tough. I don't think they'll miss departed cornerback Ken Lucas much; Jon Beason is a monster in the middle; and defensive tackles Damione Lewis and Maake Kemoeatu are underrated. … The Packers are changing to a 3-4 defense under new defensive coordinator Dom Capers, something that likely has their best defensive player, Aaron Kampman, none too pleased. As a strongside linebacker, Kampman probably won't be a blitz specialist and could see his sack totals decrease, but generally speaking, this should be a pretty good transition. Rookie B.J. Raji can handle the nose tackle position as a two-gapper right away, and playing more linebackers across the field should cover for weaknesses in A.J. Hawk's and Nick Collins' games. The real question, though, is whether aging corners Charles Woodson and Al Harris can hold up.

Falling Down

The Bears' D might have finished sixth in fantasy points last season, but there are warning signs. For the first time since 2002, this defense finished in the NFL's bottom half in both yards and points allowed. They forced the fewest fumbles in the league last year. And the defensive line just wasn't very good. Sure, Brian Urlacher and Lance Briggs can make plays, but they need a clean field in front of them, and guys like Tommie Harris, Alex Brown and Adewale Ogunleye didn't provide it in '08. I'm also worried about Nathan Vasher in the secondary. I don't view this unit as a top-10 group for '09. … Shawne Merriman will be back for the Chargers, and I sure hope he brings some pass coverage with him. San Diego allowed the second-most passing yards in the NFL last year, and not all of that can be chalked up to Merriman's absence due to torn knee ligaments. Cornerback Quentin Jammer is solid but isn't an elite cover man anymore, and '07 sensation Antonio Cromartie was regularly roasted (perhaps because of injury?) in '08. If everyone's healthy and rookie Larry English provides some pass-rush help, maybe the Chargers recapture past glory. I wouldn't draft them high just to find out, though. … Tony Dungy left the Colts, and some of the principles of his Tampa 2 defense seem to have gone with him. At least it looks that way, as Indianapolis has focused on getting bigger at linebacker, thereby stopping the run more (this defense allowed more than 122 rush yards per game in '08). But with the light, quick personnel the Colts have been using for years, there's only so much they can do in a single offseason. In most other respects (turnovers and sacks, in particular), this is a solid unit. But because it can get crunched by good running opponents, I wouldn't plan on using it as a starter. … The Buccaneers actually had a more-than-decent defensive season in '08, finishing ninth in yards allowed and 10th in points allowed. But it now seems that was a last hurrah. New head coach Raheem Morris decided not to re-sign Derrick Brooks, Cato June, Jovan Haye or Phillip Buchanon. Morris is moving safety Jermaine Phillips to linebacker, and he's going with a combo of Ryan Sims and Chris Hovan in the middle of the D-line. I don't like it. One gets a sense the Bucs might think they've got a mulligan coming in '09, as they consider starting a rookie (Josh Freeman) at quarterback. That will only put more pressure on a defense with an awful lot of moving parts.

Sleepers

The Texans have a lot of young talent on defense. Of course, they had a lot of young talent on defense last year, too, and yet they finished as fantasy's 28th-best defense. What makes me think this year will be any different? Well, they did sign Antonio Smith, who had a huge playoff run in '08, from the Cardinals, and they drafted high-motor linebacker Brian Cushing. I also like lead corner Dunta Robinson a lot (though he held out of OTAs because he's unhappy with his contract), and I expect nose tackle Amobi Okoye to be a lot better this year. More sacks and turnovers will come if Smith works out and Robinson reports. … Mike Singletary hints that the 49ers will get it done with a conservative offensive approach that doesn't put a ton of pressure on the defense. That sounds good to me. The Niners finished only 25th in fantasy points overall last year, but things got way better after Singletary and defensive coordinator Greg Manusky took over and installed a 3-4. There's enough linebacking talent here, even beyond Patrick Willis, to produce a heck of a lot more sacks than the 30 they racked up in '08.

We have the Seahawks rated just 20th, and I understand that: The offense looks like it could be a mess; Patrick Kerney is coming off major shoulder surgery; and the defensive line lost its primary run-stuffer in Rocky Bernard. But the team did draft Aaron Curry with the No. 4 overall pick this April, creating a fantastic linebacking corps with Lofa Tatupu and Leroy Hill also there. The Seahawks signed Colin Cole and traded for Cory Redding to get some depth up front. And they're hoping Ken Lucas can be far less flammable than Kelly Jennings was at corner last year. There are some very interesting players here. … After their surprise Super Bowl run, it's tempting to view the Cardinals' D as a possible fantasy starter. After all, there are names you know here: Karlos Dansby, Adrian Wilson and Darnell Dockett. Plus with ex-Steeler Bryant McFadden, they upgraded at the corner spot opposite Dominique Rodgers-Cromartie. But the rub with this team will always be that the offense moves so quickly and scores so many points that the defense has a lot of undue pressure on it. The Cardinals can play well and still not produce great fantasy stats, but it's possible they could put a better statistical season together.

Draft Strategy

Don't take your fantasy defense early. Ever. It's a big mistake to think that just because you've filled out your starting lineup at quarterback, running back, wide receiver and tight end, you should go ahead and grab your starting defense. No. You don't have to get one of the consensus preseason top defensive units. In fact, history has shown that those consensus preseason top defensive units have a really good chance of stinking, at least in terms of fantasy. Use your middle-round picks buying lottery-ticket rushers and receivers. Then toward the end of your draft, grab a supposedly middle-of-the-road defense. If you select unwisely, it doesn't matter, since there will always be a ton of fine defenses on the waiver wire. And don't draft a second defense to fill in during your first defense's bye week. You'll figure something out midseason.

Auction Strategy

I won't go so far as to say you must spend just one dollar on your fantasy defense, as I do say when I'm discussing fantasy kickers. But don't go much higher. If someone goes hog-wild early and bids $7 for the Steelers' D, just tip your cap and be glad it isn't you. As I've said several times here, there's enough uncertainty when it comes to predicting team defenses that you're better off spending as little dough as possible. Spend $2 or $3, and that's it. And unless your league mandates it, don't purchase two defenses.

Christopher Harris is a fantasy analyst for ESPN.com. He is a six-time Fantasy Sports Writers Association award winner. You can e-mail him here.