Detroit? San Francisco? NY Giants?
I'll be a little on the contrary side (at least for NY and SF) and say realistically all three you mentioned could falter, IMO.
Both the Giants and Lions play in divisions which I feel are going to be competitive in 2012.
The Eagles, Cowboys and Redskins all got better (on paper) and the Giants need to actually maintain team health and improve consistency (i.e. stop playing down to teams you should beat). Even in a down year, the division is always competitive and there hasn't been a back to back division winner since 2003-2004 (Eagles I believe). The math is against my Giants but then it was during the Super Bowl run last season as well.
The Lions have to deal with a Packers team that I think will come out with an improved defense and already has an explosive offense that's one of the best in the NFL. The Bears will get key pieces back and prior to the loss of Cutler and Forte - they were in contention for the playoffs. I might argue the Lions may not have made the playoffs if the Bears had remained healthy (not a slight to Lions fans, just a realistic appraisal of a possible outcome).
With the Niners, the West is still a fairly weak division but teams ARE improving. The Seahawks were putting it together at the end of last season and if they can find a QB, they might actually be decent. The Cardinals have a few too many holes but had a pretty decent defense and Skelton played well in spurts. The Rams are in complete rebuilding mode so they are up in the air and it will likely take time for them to be relevant.
The biggest problem I see for the Niners is that they had some really good luck in maintaining health on the defensive side of the ball. They were also leaders in not turning the ball over and getting takeaways. Both are difficult to maintain (especially the lack of injuries) and that luck can easily turn to disaster.
I do think the Niners are the least likely to falter (i.e. not make the playoffs) due to the relative strength of the opponents in their division though.