I don't think it will be Seattle. They lost a fair bit in the off-season, but they still have a very good team. 49ers won't be overly strong on D but the offense should be much better.
In the end i think it comes down to the team that stays healthiest. Injuries can decimate any team in the blink of an eye. NFC is stacked with Philly, New Orleans , Chicago, Detroit, maybe Dallas and the Redskins might sneak up on people, but that might be a stretch.
IN the AFC, i like Denver again. Other front runners are the Chiefs, Colts, Pats and Chargers.