Kinda off topic, but IMO they 49ers are easily the best defense in the NFL. It seems like every play they are stripping the ball, or their safeties are jumping a route to make an interception. Their D-Line is league-class, often getting pressure with only 3 rushers which allows for their aggressive play in the secondary. When they bring 4 (Smith, Smith, Sopoaga, McDonald) they are a force. Their D-line also allows the LB tandem of Bowman an Willis to absolutely stifle the run. Really the only way they could significantly improve is by replacing Culliver with an elite cover corner.
They allowed 2 rushing TD's all season, and 1 100- yard rusher. By far the best run defense in NFL history.
Other teams in the conversation are the Texans, Ravens, Bears, Bengals and Steelers. Notice they are all playoff teams, or were serious contenders. The focus on the NFL these days seems to be on offense (and more specifically on the QB position, and rightly so, no position influences a teams success like a QB). However a defensive unit operating on a high level puts offenses in position to win games. As much as the 49ers have improved on offense this year, I would be kidding myself if i said a big factor in that wasn't their defensive play.