Many times defense gets overlooked when selecting a fantasy team. Some people wait until the end of the draft and take whats left while others may pick two defenses and plan for their bye week. Either strategy can be effective but the most successful teams usually have a defensive plan in drafting.
When looking for that perfect defense, it's not always the best idea to take the best defense in the league. You have to factor in a lot of things when picking a solid defense because special teams are a big part of the game plan as well.
For years if you got the Ravens defense you were set. Then Devin Hester got drafted by the Chicago Bears, and even though they didn't get a ton of turnovers, they were golden because Hester was amost good for a special teams touchdown every other game. You also want a team that gets sacks. Plenty of them. They add up quickly.
The golden rule is that you never pick a defense before round 6. However someone in your league will jump the gun and try to get one of the top projected defenses. Don't panic, there are plenty of good defenses to choose from.
San Francisco 49ers
New York Jets
Green Bay Packers
New York Giants